Compositions and methods for determining resistance to inhibitors of virus entry using recombinant virus assays

ABSTRACT

The invention provides a method for determining whether a human immunodeficiency virus is resistance to a viral entry inhibitor. The methods are particularly useful for determining resistance to inhibitors that act by a non-competitive mechanism. In certain aspects, the methods comprise determining whether an HIV population is resistant to an HIV entry inhibitor, comprising determining a log-sigmoid inhibition curve comprising data points for entry of the HIV population in the presence of varying concentrations of the HIV entry inhibitor, wherein if the entry of the HIV population cannot be completely inhibited by the HIV entry inhibitor, the HIV population is resistant to the HIV entry inhibitor.

The present application is a continuation of U.S. patent applicationSer. No. 11/146,879, filed Jun. 6, 2005 (allowed), which is entitled toand claims benefit of U.S. Provisional Application No. 60/577,851, filedJun. 7, 2004. The contents of both of these applications are herebyincorporated by reference in their entirety. Each of these applicationsis hereby incorporated by reference in its entirety.

1. CROSS-REFERENCE TO RELATED APPLICATION AND STATEMENT OF POTENTIALGOVERNMENT RIGHTS

The invention described in this application was supported, in part, bySmall Business Innovation Research (SBIR) Grant R44 AI048990. The UnitedStates government may have certain rights to this invention.

Throughout this application, various publications are referenced byauthor and date within the text. Full citations for these publicationsmay be found listed alphabetically at the end of the specificationimmediately preceding the claims. All patents, patent applications andpublications cited herein, whether supra or infra, are herebyincorporated by reference in their entirety. The disclosures of thesepublications in their entireties are hereby incorporated by referenceinto this application in order to more fully describe the state of theart as known to those skilled therein as of the date of the inventiondescribed and claimed herein.

2. BACKGROUND

Enveloped animal viruses attach to and enter the host cell via theinteraction of viral proteins in the virion membrane (envelope proteins)and cell surface proteins (virus receptors). Receptor recognition andbinding are mediated by the surface envelope protein. Virus entry is anattractive target for anti-viral treatment; numerous drugs that aredesigned to block virus attachment or membrane fusion have been or arecurrently being evaluated in preclinical or clinical studies (Richman,1998; PhRMA, 1999; Stephenson, 1999). For example, the attachmentinhibitor SCH-D, which blocks the interaction between viral membraneproteins and CCR5 is currently being evaluated in clinical studies forits effectiveness as an anti-viral treatment (Schurmann, D. et al.,2004). Other entry inhibitors currently under investigation includeUK-427857 (Pfizer), TNX-355 (Tanox Inc.), AMD-070 (AnorMED), Pro 140(Progenies), FP-21399 (EMD Lexigen), BMS-488043 (Bristol-Myers Squibb),and GSK-873,140 (GlaxoSmithKline) One entry inhibitor, T-20(Roche/Trimeris), has been approved to treat HIV infection.

As these drugs continue to be developed and enter the clinic, assays areneeded that can rapidly and easily detect the emergence of viruses withreduced susceptibility to entry inhibitors. In particular, methods fordetermining whether an HIV is resistant to an entry inhibitor, e.g.,SCH-C, SCH-D, UK-427857 and/or TNX-355, are needed. These and otherunmet needs are provided by the present invention.

3. SUMMARY

In certain aspects, the invention provides a method for determiningwhether an human immunodeficiency virus (“HIV”) is resistant to an HIVentry inhibitor. In one aspect, the invention provides a method fordetermining whether an human immunodeficiency virus (“HIV”) is resistantto an HIV entry inhibitor that comprises generating a log-sigmoidinhibition curve comprising data points that measure entry of the HIVinto a cell in the presence of varying concentrations of the HIV entryinhibitor; and comparing the inhibition curve of step (a) to alog-sigmoid inhibition curve for a reference HIV. In certainembodiments, the curve for the reference virus at or near the same timeas the log-sigmoid curve of the HIV. In certain embodiments, the curvefor the reference virus can be a known, standard curve for the referencevirus. In certain embodiments, a decrease in the maximum inhibitionpercentage observed for the HIV relative to that observed for thereference HIV indicates that the HIV is resistant to the HIV entryinhibitor. In certain embodiments, the entry inhibitor is selected fromthe group consisting of SCH-C, SCH-D, UK-427857, and TNX-355. In certainembodiments, the reference HIV is HXB2, NL4-3, or SF2. In certainembodiments, the HIV is from a subject infected with HIV.

In certain embodiments, the data points that measure entry of the HIVinto the cell are determined by contacting an HIV viral particle withthe cell in the presence of the HIV entry inhibitor, wherein the cellexpresses a cell surface receptor to which the viral particle binds, andwherein the viral particle comprises: (i) a viral expression vector thatlacks a nucleic acid encoding a functional viral envelope protein, butwhich comprises an indicator nucleic acid that produces a detectablesignal when introduced into the cell, and (ii) a viral envelope proteinencoded by a nucleic acid of the HIV; and measuring the amount of thedetectable signal produced by the cell. In certain embodiments, thedetectable signal is a fluorescent signal. In certain embodiments, theindicator nucleic acid encodes luciferase. In certain embodiments, theviral particle is produced by co-transfecting into a cell (i) a nucleicacid encoding a viral envelope protein of the HIV, and (ii) a viralexpression vector lacking a nucleic acid encoding an envelope protein,wherein the vector comprises an indicator nucleic acid that produces adetectable signal. In certain embodiments, the cell surface receptor isCD4. In certain embodiments, the cell also expresses a chemokinereceptor. In certain embodiments, the chemokine receptor is CXCR4 orCCR5. In certain embodiments, the HIV nucleic acid encodes gp160, gp120,and/or gp41.

In other aspects, the invention provides a method for determiningwhether an HIV population is resistant to an HIV entry inhibitor,wherein the method comprises generating a log-sigmoid inhibition curvecomprising data points that measure entry of the HIV population into acell in the presence of varying concentrations of the HIV entryinhibitor; and comparing the inhibition curve of step (a) to alog-sigmoid inhibition curve for a reference HIV population. In certainembodiments, a decrease in the maximum inhibition percentage observedfor the HIV population relative to that observed for the reference HIVpopulation indicates that the HIV is resistant to the HIV entryinhibitor.

In other aspects, the invention provides a method for determiningwhether an HIV is resistant to an HIV entry inhibitor that comprisesdetermining a log-sigmoid inhibition curve comprising data points forentry of the HIV in the presence of varying concentrations of the HIVentry inhibitor, wherein if the entry of the HIV cannot be completelyinhibited by the HIV entry inhibitor, the HIV is resistant to the HIVentry inhibitor.

In another aspect, the invention provides a method for determiningwhether an HIV population is resistant to an HIV entry inhibitor,comprising determining a log-sigmoid inhibition curve comprising datapoints for entry of the HIV population in the presence of varyingconcentrations of the HIV entry inhibitor, wherein if the entry of theHIV population cannot be completely inhibited by the HIV entryinhibitor, the HIV population is resistant to the HIV entry inhibitor.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A: Structure of Envelope Expression and Viral Expression Vectors.

The HIV envelope expression vector (pHIVenv) is modified to acceptenvelope sequences that have been amplified from subject plasma samples.The designations a/b and c/d, refer to restriction endonuclease sitespositioned at the 5′ and 3′ end of the HIV-1 envelope polyprotein(gp160). The HIV expression vector (pHIVlucΔU3) encodes all HIV proteinsexcept the envelope polyprotein. A portion of the envelope gene has beendeleted to accommodate a indicator gene cassette, in this case, fireflyluciferase that is used to monitor the ability of the virus to replicatein the presence or absence of anti-viral drugs. The 3′ U3 region hasbeen partially deleted to prevent transcription from the 5′ LTR ininfected cells. Virus produced in this system is limited to a singleround of replication.

FIG. 1B: Cell Based Entry Assay

Drug susceptibility, co-receptor tropism and virus neutralizationtesting are performed by co-transfecting a host cell with pHIVenv andpHIVlucΔU3. The host cell produces HIV particles that are pseudo-typedwith HIV envelope sequences derived from the test virus or subjectsample. Virus particles are collected (.about.48 h) after transfectionand are used to infect target cells that express HIV receptors (e.g.CD4) and co-receptors (e.g. CXCR4, CCR5). After infection (.about.72 h)the target cells are lysed and luciferase activity is measured. HIV mustcomplete one round of replication to successfully infect the target hostcell and produce luciferase activity. If the virus is unable to enterthe target cell, luciferase activity is diminished. This system can beused to evaluate susceptibility to entry inhibitors, receptor andco-receptor tropism, and virus neutralization.

FIG. 2: HIV Envelope Expression Vectors.

HIV envelope sequences are amplified from subject samples and insertedinto expression vectors using restriction endonuclease sites (5′ a/b and3′ c/d). Envelope transcription is driven by the immediate early genepromoter of human cytomegalovirus (CMV). Envelope RNA is polyadenylatedusing an simian virus 40 (SV40) polyadenylation signal sequence (A+). Anintron located between the CMV promoter and the HIV envelope sequencesis designed to increase envelope mRNA levels in transfected cells.FL-express full-length envelope proteins (gp120, gp41); ΔCT-expressenvelope proteins (gp120, gp41) lacking the C-terminal cytoplasmic taildomain of gp41; +CT-express envelope proteins (gp120, gp41) containing aconstant pre-defined gp41 cytoplasmic tail domain; gp120-express gp120proteins derived from the subject together with a constant pre-definedgp41; and gp41-express a constant pre-defined gp120 together with gp41proteins derived from the subject.

FIG. 3A: Co-receptor Tropism Screening Assay.

In this figure, the assay is performed using two cell lines. One cellline expresses CD4 and CCR5 (top six panels). The other cell lineexpresses CD4 and CXCR4 (bottom six panels). The assay is performed byinfecting cells with a large number of recombinant virus stocks derivedfrom cells transfected with pHIVenv and pHIVlucΔU3 vectors. The exampleshown represents the analysis of 96 viruses formatted in a 96 wellplate. Infections are performed in the absence of drug (no drug), or inthe presence of a drug that preferentially inhibits either R5 tropic(CCR inhibitor) or X4 tropic (CXCR4 inhibitor) viruses. Co-receptortropism is assessed by comparing the amount of luciferase activityproduced in each cell type, both in the presence and absence of drug(see FIG. 3B for interpretation of assay results).

FIG. 3B: Determining Co-Receptor Tropism.

In this figure, the results of the assay are interpreted by comparingthe ability of each sample virus to infect (produce luciferase activity)cells expressing CD4/CCR5 (R5 cells) or cells expressing CD4/CXCR4 (X4cells). The ability of a CCR5 or CXCR4 inhibitor to specifically blockinfection (inhibit luciferase activity) is also evaluated. X4 tropicviruses (green panels)—infect cells expressing CXCR4 but not cellsexpressing CCR5. Infection of X4 cells is blocked by the CXCR4inhibitor. R5 tropic viruses (blue panels)-infect R5 cells but not X4cells. Infection of R5 cells is blocked by the CCR5 inhibitor. Dualtropic or X4/R5 mixtures (yellow panels) infect X4 and R5 cells.Infection of R5 cells is blocked by the CCR5 inhibitor and infection ofX4 cells is blocked by the CXCR4 inhibitor. Non-viable viruses (redpanels)—do not replicate in either X4 or R5 cells.

FIG. 4A: Measuring Entry Inhibitor Susceptibility: Fusion Inhibitor.

In this figure, susceptibility to the fusion inhibitor T-20 isdemonstrated. Cells expressing CD4, CCR5 and CXCR4 were infected in theabsence of T-20 and over a wide range of T-20 concentrations α-axis log10 scale). The percent inhibition of viral replication (y-axis) wasdetermined by comparing the amount of luciferase produced in infectedcells in the presence of T-20 to the amount of luciferase produced inthe absence of T-20. R5 tropic, X4 tropic and dual tropic viruses weretested. Drug susceptibility is quantified by determining theconcentration of T-20 required to inhibit 50% of viral replication(IC₅₀, shown as vertical dashed lines). Viruses with lower IC₅₀ valuesare more susceptible to T-20 than viruses with higher IC₅₀ values.NL4-3: well-characterized X4 tropic strain; JRCSF: well-characterized R5tropic strain; 91US005.11: R5 tropic isolate obtained from the NIH AIDSResearch and Reference Reagent Program (ARRRP); 92HT593.1: Dual tropic(X4R5) isolate obtained from the NIH ARRRP; and 92HT599.24: X4 tropicisolate obtained from the NIH ARRRP.

FIG. 4B: Measuring Entry Inhibitor Susceptibility: Drug ResistanceMutations.

In this figure, reduced susceptibility to the fusion inhibitor T-20conferred by specific drug resistance mutations in the gp41 envelopeprotein is demonstrated. Cells expressing CD4, CCR5 and CXCR4 wereinfected in the absence of T-20 and over a wide range of T-20concentrations (x-axis log 10 scale). The percent inhibition of viralreplication (y-axis) was determined by comparing the amount ofluciferase produced in infected cells in the presence of T-20 to theamount of luciferase produced in the absence of T-20. Isogenic virusescontaining one or two specific mutations in the gp41 transmembraneenvelope protein were tested (highlighted in red in the figure legend).Drug susceptibility is quantified by determining the concentration ofT-20 required to inhibit 50% of viral replication (IC₅₀, shown asvertical dashed lines). Viruses with lower IC₅₀ values are moresusceptible to T-20 than viruses with higher IC₅₀ values. No mutation(wildtype sequence): GIV; Single mutations: DIV, DIM, SIV; Doublemutations: DIM, SIM, DTV.

FIG. 5: Competitive Entry Inhibitor Resistance.

FIG. 5 demonstrates log-sigmoid curves for viruses resistant to AMD-3100and T20, indicating that resistance to these entry inhibitors manifestsas an increased IC₅₀ and that entry can be completely inhibited withhigh concentrations of inhibitor.

FIG. 6: Non-Competitive Entry Inhibitor Resistance.

FIG. 6 demonstrates log-sigmoid curves for viruses resistant to SCH-Cand TNX-355, indicating that resistance to these entry inhibitorsmanifests as a reduced maximum percentage of inhibition.

FIG. 7: Mixed Susceptible and Resistant Viral Envelope Proteins.

FIG. 7 presents the results of DNA mixing experiments that demonstratelog-sigmoid curves for mixed resistant and sensitive envelope proteins.Briefly, the proportion of resistant and sensitive envelope proteinswere varied as presented in the legend of the figures and the percentageinhibition observed was plotted against log 10 of inhibitorconcentration. The results indicate that susceptibility or resistance ofmixed populations of viruses to entry inhibitors can also be assessedaccording to the present invention.

FIG. 8: Noncompetitive Entry Inhibitor Resistance.

FIG. 8 demonstrates log-sigmoid curves for viruses resistant toUK-427,857, indicating that resistance to this entry inhibitor manifestsas a reduced maximum percentage of inhibition.

FIG. 9: Reproducibility of Inhibition Curve Plateaus for aNoncompetitive Entry Inhibitor.

FIG. 9 shows the reproducibility of the maximum percentage of inhibitionobserved for a representative noncompetitive entry inhibitor,UK-427,857. As show in FIG. 9, multiple measurements of the maximumpercentage of inhibition cluster reproducibly.

5. DEFINITIONS

As used herein, the following terms shall have the following meanings.

A “phenotypic assay” is a test that measures a phenotype of a particularvirus, such as, for example, HIV, or a population of viruses, such as,for example, the population of HIV infecting a subject. The phenotypesthat can be measured include, but are not limited to, the resistance orsusceptibility of a virus, or of a population of viruses, to a specificanti-viral agent or that measures the replication capacity of a virus.

A “genotypic assay” is an assay that determines a genotype of anorganism, a part of an organism, a population of organisms, a gene, apart of a gene, or a population of genes. Typically, a genotypic assayinvolves determination of the nucleic acid sequence of the relevant geneor genes. Such assays are frequently performed in HIV to establish, forexample, whether certain mutations are associated with drug resistanceor resistance or altered replication capacity are present. Genotypes areroutinely described as the derived protein sequence encoded by the geneor genes of interest.

The term “% sequence identity” is used interchangeably herein with theterm “% identity” and refers to the level of amino acid sequenceidentity between two or more peptide sequences or the level ofnucleotide sequence identity between two or more nucleotide sequences,when aligned using a sequence alignment program. For example, as usedherein, 80% identity means the same thing as 80% sequence identitydetermined by a defined algorithm, and means that a given sequence is atleast 80% identical to another length of another sequence. Exemplarylevels of sequence identity include, but are not limited to, 60, 70, 80,85, 90, 95, 98% or more sequence identity to a given sequence.

The term “% sequence homology” is used interchangeably herein with theterm “% homology” and refers to the level of amino acid sequencehomology between two or more peptide sequences or the level ofnucleotide sequence homology between two or more nucleotide sequences,when aligned using a sequence alignment program. For example, as usedherein, 80% homology means the same thing as 80% sequence homologydetermined by a defined algorithm, and accordingly a homologue of agiven sequence has greater than 80% sequence homology over a length ofthe given sequence. Exemplary levels of sequence homology include, butare not limited to, 60, 70, 80, 85, 90, 95, 98% or more sequencehomology to a given sequence.

Exemplary computer programs which can be used to determine identitybetween two sequences include, but are not limited to, the suite ofBLAST programs, e.g., BLASTN, BLASTX, and TBLASTX, BLASTP and TBLASTN,publicly available on the Internet at the NCBI website. See alsoAltschul et al., 1990, J. Mol. Biol. 215:403-10 (with special referenceto the published default setting, i.e., parameters w=4, t=17) andAltschul et al., 1997, Nucleic Acids Res., 25:3389-3402. Sequencesearches are typically carried out using the BLASTP program whenevaluating a given amino acid sequence relative to amino acid sequencesin the GenBank Protein Sequences and other public databases. The BLASTXprogram is preferred for searching nucleic acid sequences that have beentranslated in all reading frames against amino acid sequences in theGenBank Protein Sequences and other public databases. Both BLASTP andBLASTX are run using default parameters of an open gap penalty of 11.0,and an extended gap penalty of 1.0, and utilize the BLOSUM-62 matrix.See id.

A preferred alignment of selected sequences in order to determine “%identity” between two or more sequences, is performed using for example,the CLUSTAL-X program, operated with default parameters, including anopen gap penalty of 10.0, an extended gap penalty of 0.1, and a BLOSUM30 similarity matrix.

“Polar Amino Acid” refers to a hydrophilic amino acid having a sidechain that is uncharged at physiological pH, but which has at least onebond in which the pair of electrons shared in common by two atoms isheld more closely by one of the atoms. Genetically encoded polar aminoacids include Asn (N), Gln (O) Ser (S) and Thr (T).

“Nonpolar Amino Acid” refers to a hydrophobic amino acid having a sidechain that is uncharged at physiological pH and which has bonds in whichthe pair of electrons shared in common by two atoms is generally heldequally by each of the two atoms (i.e., the side chain is not polar).Genetically encoded nonpolar amino acids include Ala (A), Gly (G), Ile(I), Leu (L), Met (M) and Val (V).

“Hydrophilic Amino Acid” refers to an amino acid exhibiting ahydrophobicity of less than zero according to the normalized consensushydrophobicity scale of Eisenberg et al., 1984, J. Mol. Biol.179:125-142. Genetically encoded hydrophilic amino acids include Arg(R), Asn (N), Asp (D), Glu (E), Gln (O), H is (H), Lys (K), Ser (S) andThr (T).

“Hydrophobic Amino Acid” refers to an amino acid exhibiting ahydrophobicity of greater than zero according to the normalizedconsensus hydrophobicity scale of Eisenberg et al., 1984, J. Mol. Biol.179:125-142. Genetically encoded hydrophobic amino acids include Ala(A), Gly (G), Ile (I), Leu (L), Met (M), Phe (F), Pro (P), Trp (W), Tyr(Y) and Val (V).

“Acidic Amino Acid” refers to a hydrophilic amino acid having a sidechain pK value of less than 7. Acidic amino acids typically havenegatively charged side chains at physiological pH due to loss of ahydrogen ion. Genetically encoded acidic amino acids include Asp (D) andGlu (E).

“Basic Amino Acid” refers to a hydrophilic amino acid having a sidechain pK value of greater than 7. Basic amino acids typically havepositively charged side chains at physiological pH due to associationwith a hydrogen ion. Genetically encoded basic amino acids include Arg(R), H is (H) and Lys (K).

A “mutation” is a change in a nucleic acid sequence or in acorresponding amino acid sequence relative to a reference nucleic acidor polypeptide. For embodiments of the invention comprising HIV proteaseor reverse transcriptase, the reference nucleic acid encoding proteaseor reverse transcriptase is the protease or reverse transcriptase codingsequence, respectively, present in NL4-3 HIV (GenBank Accession No.AF324493). Likewise, the reference protease or reverse transcriptasepolypeptide is that encoded by the NL4-3 HIV sequence. Although theamino acid sequence of a peptide can be determined directly by, forexample, Edman degradation or mass spectroscopy, more typically, theamino sequence of a peptide is inferred from the nucleotide sequence ofa nucleic acid that encodes the peptide. Any method for determining thesequence of a nucleic acid known in the art can be used, for example,Maxam-Gilbert sequencing (Maxam et al., 1980, Methods in Enzymology65:499), dideoxy sequencing (Sanger et al., 1977, Proc. Natl. Acad. Sci.USA 74:5463) or hybridization-based approaches (see e.g., Sambrook etal., 2001, Molecular Cloning: A Laboratory Manual, Cold Spring HarborLaboratory, 3^(rd) ed., NY; and Ausubel et al., 1989, Current Protocolsin Molecular Biology, Greene Publishing Associates and WileyInterscience, NY).

A “mutant” is a virus, gene or protein having a sequence that has one ormore changes relative to a reference virus, gene or protein.

The terms “peptide,” “polypeptide” and “protein” are usedinterchangeably throughout.

The term “wild-type” refers to a viral genotype that does not comprise amutation known to be associated with drug resistance.

The terms “polynucleotide,” “oligonucleotide” and “nucleic acid” areused interchangeably throughout.

6. DETAILED DESCRIPTION OF THE INVENTION

In certain aspects, the invention provides a method for determiningwhether an HIV is resistant to an HIV entry inhibitor. The methods areuseful, for example, to guide therapeutic decisions in treatmentsubjects infected with HIV, whether newly infected or in treatment,including failing treatment, and for screening compounds to identifycompounds that will inhibit viruses resistant to other entry inhibitors.Other uses of such methods will be apparent to those of skill in theart.

6.1 Methods for Determining Whether an HIV or HIV Population isResistant to Entry Inhibitors

In one aspect, the invention provides a method for determining whetheran HIV is resistant to an HIV entry inhibitor. The methods generallycomprise determining phenotypic drug susceptibility of an HIV or an HIVpopulation using, for example, the phenotypic drug susceptibility assayspresented in Example 1, and determining whether the HIV or HIVpopulation is resistant to an entry inhibitor as described hereinafter.However, any such susceptibility assay known by one of skill in the artcan in principle be used in the methods for determining phenotypic drugsusceptibility. Drug susceptibility can be plotted, for example, aspercent inhibition versus log₁₀ drug concentration and defined based on,for example, the IC₅₀ and percent inhibition at the highest drugconcentration. Such percent inhibition observed at the highest drugconcentration for a reference HIV, such as, for example, HXB2, NL4-3, orSF2, is the maximum percent inhibition (max % inhibition).

In certain embodiments, the methods can be used to identify resistanceto entry inhibitors that exhibit competitive kinetics, i.e., competewith viral proteins for access to viral or cellular components thatmediate virus entry. Such entry inhibitors include, but are not limitedto, T-20 and AMD-3100, discussed above. Viruses that are resistant tosuch entry inhibitors exhibit an increased IC₅₀ relative to susceptibleviruses.

In other embodiments, the methods can be used to identify resistance toentry inhibitors that exhibit non-competitive kinetics, i.e., do notdirectly compete with viral proteins for access to viral or cellularcomponents that mediate entry, but instead alter the conformation of thecellular component and/or viral protein in a manner that disrupts theinteraction between such proteins and components. Examples of suchnon-competitive inhibitors include, but are not limited to, entryinhibitors such as SCH-C, TNX-355, UK-427,857, SCH-D, GW-873,140, andTAK-220. For the first time, Applicants have identified that virusesresistant to such entry inhibitors exhibit non-competitive kinetics forinhibition.

Viruses resistant to such non-competitive entry inhibitors manifest suchresistance in at least two different ways. First, viruses resistant tonon-competitive entry inhibitors cannot be completely inhibited. Thatis, there is no concentration of entry inhibitor that can completelyinhibit the envelope protein-cell surface receptor interaction. Thus,when a log-sigmoid curve is plotted comparing a measurement of virusentry to drug concentration, the curve plateaus at less than 100%inhibition. As such, where a log-sigmoid curve generated as describedherein indicates that no matter how much entry inhibitor is added to theassay mixture, the virus remains able to enter the cell at a detectablelevel, the virus is resistant to the entry inhibitor.

Second, such viruses exhibit reduced maximum percentages of inhibitionrelative to susceptible viruses. Thus, by observing that a particularvirus or viral population exhibits a reduced maximum percentage ofinhibition relative to a reference virus or viral population, it can bedetermined that the virus or viral population exhibits reducedsusceptibility (resistance) to the tested entry inhibitor. The referencevirus or viral population can be a virus or viral population obtainedfrom a subject or other source prior to therapy with or exposure to anentry inhibitor. Alternatively, the reference virus or viral populationcan be a reference strain, such as, for example, SF2, HXB2, or NL4-3. Asyet another alternative, the maximum percentage of inhibition observedfor the reference virus or viral population can be an average maximumpercentage of inhibition derived from a number of viruses or viralpopulations.

Thus, in certain embodiments, the invention provides a method fordetermining whether an HIV is resistant to an HIV entry inhibitor thatcomprises generating a log-sigmoid inhibition curve comprising datapoints that measure entry of the HIV into a cell in the presence ofvarying concentrations of the HIV entry inhibitor; and comparing theinhibition curve of step (a) to a log-sigmoid inhibition curve for areference HIV. In certain embodiments, a decrease in the maximuminhibition percentage observed for the HIV relative to that observed forthe reference HIV indicates that the HIV is resistant to the HIV entryinhibitor.

In certain embodiments, the entry inhibitor is selected from the groupconsisting of BMS-488,403 (Bristol-Meyers Squibb; New York, N.Y.),Pro-542 (Progenics Pharmaceuticals, Inc.; Tarrytown, N.Y.), mAb B4(United BioMedical, Inc.; Hauppauge, N.Y.), TNX-355 (Tanox Inc.;Houston, Tex.), UK-427,857 (Pfizer Inc.; New York, N.Y.), SCH-D(Schering-Plough; Kenilworth, N.J.), GW-873,140 (GlaxoSmithKline;Research Triangle, N.C.), AMD-11070 (AnorMED Inc.; Langley, Canada),TAK-220 (Takeda Chemical Industries, Ltd.; Osaka, Japan), Pro-140(Progenics Pharmaceuticals, Inc.; Tarrytown, N.Y.), and mAb004 (HumanGenome Sciences Inc.; Rockville, Md.). In certain embodiments, the entryinhibitor is selected from the group consisting of TNX-355, UK-427,857,SCH-D, GW-873,140, AMD-11070, and TAK-220. In certain embodiments, theentry inhibitor is selected from the group consisting of TNX-355,UK-427,857, SCH-D, GW-873,140, and TAK-220. In certain embodiments, theentry inhibitor is selected from the group consisting of SCH-C, SCH-D,UK-427857, and TNX-355. In certain embodiments, the entry inhibitor isBMS-488,403. In certain embodiments, the entry inhibitor is Pro-542. Incertain embodiments, the entry inhibitor is mAb B4. In certainembodiments, the entry inhibitor is TNX-355. In certain embodiments, theentry inhibitor is UK-427,857. In certain embodiments, the entryinhibitor is SCH-D. In certain embodiments, the entry inhibitor isGW-873,140. In certain embodiments, the entry inhibitor is AMD-11070. Incertain embodiments, the entry inhibitor is TAK-220. In certainembodiments, the entry inhibitor is Pro-140. In certain embodiments, theentry inhibitor is mAb004. In certain embodiments, the reference HIV isHXB2, NL4-3, or SF2. In certain embodiments, the HIV is from a subjectinfected with HIV.

In certain embodiments, the data points that measure entry of the HIVinto the cell are determined by contacting an HIV viral particle withthe cell in the presence of the HIV entry inhibitor, wherein the cellexpresses a cell surface receptor to which the viral particle binds, andwherein the viral particle comprises: (i) a viral expression vector thatlacks a nucleic acid encoding a viral envelope protein, but whichcomprises an indicator nucleic acid that produces a detectable signalwhen introduced into the cell, and (ii) a viral envelope protein encodedby a nucleic acid of the HIV; and measuring the amount of the detectablesignal produced by the cell. In certain embodiments, the detectablesignal is a fluorescent signal. In certain embodiments, the indicatornucleic acid encodes luciferase. In certain embodiments, the viralparticle is produced by co-transfecting into a cell (i) a nucleic acidencoding a viral envelope protein of the HIV, and (ii) a viralexpression vector lacking a nucleic acid encoding an envelope protein,wherein the vector comprises an indicator nucleic acid that produces adetectable signal. In certain embodiments, the cell surface receptor isCD4. In certain embodiments, the cell also expresses a chemokinereceptor. In certain embodiments, the chemokine receptor is CXCR4 orCCR5. In certain embodiments, the HIV nucleic acid encodes gp160, gp120,or gp41.

In other aspects, the invention provides a method for determiningwhether an HIV population is resistant to an HIV entry inhibitor thatcomprises generating a log-sigmoid inhibition curve comprising datapoints that measure entry of the HIV population into a cell in thepresence of varying concentrations of the HIV entry inhibitor; andcomparing the inhibition curve of step (a) to a log-sigmoid inhibitioncurve for a reference HIV population. In certain embodiments, a decreasein the maximum inhibition percentage observed for the HIV populationrelative to that observed for the reference HIV population indicatesthat the HIV is resistant to the HIV entry inhibitor.

In certain embodiments, the entry inhibitor is selected from the groupconsisting of SCH-C, SCH-D, UK-427857, and TNX-355. In certainembodiments, the entry inhibitor is selected from the group consistingof BMS-488,403, Pro-542, mAb B4, TNX-355, UK-427,857, SCH-D, GW-873,140,AMD-11070, TAK-220, Pro-140, and mAb004. In certain embodiments, theentry inhibitor is selected from the group consisting of TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, and TAK-220. In certainembodiments, the entry inhibitor is selected from the group consistingof TNX-355, UK-427,857, SCH-D, GW-873,140, and TAK-220. In certainembodiments, the entry inhibitor is BMS-488,403. In certain embodiments,the entry inhibitor is Pro-542. In certain embodiments, the entryinhibitor is mAb B4. In certain embodiments, the entry inhibitor isTNX-355. In certain embodiments, the entry inhibitor is UK-427,857. Incertain embodiments, the entry inhibitor is SCH-D. In certainembodiments, the entry inhibitor is GW-873,140. In certain embodiments,the entry inhibitor is AMD-11070. In certain embodiments, the entryinhibitor is TAK-220. In certain embodiments, the entry inhibitor isPro-140. In certain embodiments, the entry inhibitor is mAb004. Incertain embodiments, the reference HIV population is an HXB2, NL4-3, orSF2 population. In certain embodiments, the HIV population is from asubject infected with HIV.

In certain embodiments, the data points that measure entry of the HIVpopulation into the cell are determined by contacting a plurality of HIVviral particles with the cell in the presence of the HIV entryinhibitor, wherein the cell expresses a cell surface receptor to whichthe viral particles bind, and wherein each of the plurality of viralparticles comprises: (i) a viral expression vector that lacks a nucleicacid encoding a viral envelope protein, but which comprises an indicatornucleic acid that produces a detectable signal when introduced into thecell, and (ii) a viral envelope protein encoded by a nucleic acid of theHIV population; and measuring the amount of the detectable signalproduced by the cell. In certain embodiments, the detectable signal is afluorescent signal. In certain embodiments, the indicator nucleic acidencodes luciferase. In certain embodiments, each of the plurality of HIVviral particles comprises the same viral envelope protein. In certainembodiments, the plurality of viral particles are produced byco-transfecting into a cell (i) a plurality of nucleic acids eachencoding a viral envelope protein of the HIV population, and (ii) aviral expression vector lacking a nucleic acid encoding an envelopeprotein, wherein the vector comprises an indicator nucleic acid thatproduces a detectable signal. In certain embodiments, the cell surfacereceptor is CD4. In certain embodiments, the cell also expresses achemokine receptor. In certain embodiments, the chemokine receptor isCXCR4 or CCR5. In certain embodiments, the HIV nucleic acid encodesgp160, gp120, or gp41.

In other aspects, the invention provides a method for determiningwhether an HIV is resistant to an HIV entry inhibitor that comprisesdetermining a log-sigmoid inhibition curve comprising data points forentry of the HIV in the presence of varying concentrations of the HIVentry inhibitor, wherein if the entry of the HIV cannot be completelyinhibited by the HIV entry inhibitor, the HIV is resistant to the HIVentry inhibitor.

In certain embodiments, the entry inhibitor is selected from the groupconsisting of SCH-C, SCH-D, UK-427857, and TNX-355. In certainembodiments, the entry inhibitor is selected from the group consistingof BMS-488,403, Pro-542, mAb B4, TNX-355, UK-427,857, SCH-D, GW-873,140,AMD-11070, TAK-220, Pro-140, and mAb004. In certain embodiments, theentry inhibitor is selected from the group consisting of TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, and TAK-220. In certainembodiments, the entry inhibitor is selected from the group consistingof TNX-355, UK-427,857, SCH-D, GW-873,140, and TAK-220. In certainembodiments, the entry inhibitor is BMS-488,403. In certain embodiments,the entry inhibitor is Pro-542. In certain embodiments, the entryinhibitor is mAb B4. In certain embodiments, the entry inhibitor isTNX-355. In certain embodiments, the entry inhibitor is UK-427,857. Incertain embodiments, the entry inhibitor is SCH-D. In certainembodiments, the entry inhibitor is GW-873,140. In certain embodiments,the entry inhibitor is AMD-11070. In certain embodiments, the entryinhibitor is TAK-220. In certain embodiments, the entry inhibitor isPro-140. In certain embodiments, the entry inhibitor is mAb004. Incertain embodiments, the HIV is from a subject infected with HIV.

In certain embodiments, the data points that measure entry of the HIVinto the cell are determined by: contacting an HIV viral particle withthe cell in the presence of the HIV entry inhibitor, wherein the cellexpresses a cell surface receptor to which the viral particle binds, andwherein the viral particle comprises: (i) a viral expression vector thatlacks a nucleic acid encoding a viral envelope protein, but whichcomprises an indicator nucleic acid that produces a detectable signalwhen introduced into the cell, and (ii) a viral envelope protein encodedby a nucleic acid of the HIV; and measuring the amount of the detectablesignal produced by the cell. In certain embodiments, the detectablesignal is a fluorescent signal. In certain embodiments, the indicatornucleic acid encodes luciferase. In certain embodiments, the viralparticle is produced by co-transfecting into a cell (i) a nucleic acidencoding a viral envelope protein of the HIV, and (ii) a viralexpression vector lacking a nucleic acid encoding an envelope protein,wherein the vector comprises an indicator nucleic acid that produces adetectable signal. In certain embodiments, the cell surface receptor isCD4. In certain embodiments, the cell also expresses a chemokinereceptor. In certain embodiments, the chemokine receptor is CXCR4 orCCR5. In certain embodiments, the HIV nucleic acid encodes gp160, gp120,or gp41.

In another aspect, the invention provides a method for determiningwhether an HIV population is resistant to an HIV entry inhibitor,comprising determining a log-sigmoid inhibition curve comprising datapoints for entry of the HIV population in the presence of varyingconcentrations of the HIV entry inhibitor, wherein if the entry of theHIV population cannot be completely inhibited by the HIV entryinhibitor, the HIV population is resistant to the HIV entry inhibitor.

In certain embodiments, the entry inhibitor is selected from the groupconsisting of SCH-C, SCH-D, UK-427857, and TNX-355. In certainembodiments, the entry inhibitor is selected from the group consistingof BMS-488,403, Pro-542, mAb B4, TNX-355, UK-427,857, SCH-D, GW-873,140,AMD-11070, TAK-220, Pro-140, and mAb004. In certain embodiments, theentry inhibitor is selected from the group consisting of TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, and TAK-220. In certainembodiments, the entry inhibitor is selected from the group consistingof TNX-355, UK-427,857, SCH-D, GW-873,140, and TAK-220. In certainembodiments, the entry inhibitor is BMS-488,403. In certain embodiments,the entry inhibitor is Pro-542. In certain embodiments, the entryinhibitor is mAb B4. In certain embodiments, the entry inhibitor isTNX-355. In certain embodiments, the entry inhibitor is UK-427,857. Incertain embodiments, the entry inhibitor is SCH-D. In certainembodiments, the entry inhibitor is GW-873,140. In certain embodiments,the entry inhibitor is AMD-11070. In certain embodiments, the entryinhibitor is TAK-220. In certain embodiments, the entry inhibitor isPro-140. In certain embodiments, the entry inhibitor is mAb004. Incertain embodiments, the HIV population is from a subject infected withHIV.

In certain embodiments, the data points that measure entry of the HIVpopulation into the cell are determined by contacting a plurality of HIVviral particles with the cell in the presence of the HIV entryinhibitor, wherein the cell expresses a cell surface receptor to whichthe viral particles bind, and wherein each of the plurality of viralparticles comprises: (i) a viral expression vector that lacks a nucleicacid encoding a viral envelope protein, but which comprises an indicatornucleic acid that produces a detectable signal when introduced into thecell, and (ii) a viral envelope protein encoded by a nucleic acid of theHIV population; and measuring the amount of the detectable signalproduced by the cell. In certain embodiments, the detectable signal is afluorescent signal. In certain embodiments, the indicator nucleic acidencodes luciferase. In certain embodiments, each of the plurality of HIVviral particles comprises the same viral envelope protein. In certainembodiments, the plurality of viral particles are produced byco-transfecting into a cell (i) a plurality of nucleic acids eachencoding a viral envelope protein of the HIV population, and (ii) aviral expression vector lacking a nucleic acid encoding an envelopeprotein, wherein the vector comprises an indicator nucleic acid thatproduces a detectable signal. In certain embodiments, the cell surfacereceptor is CD4. In certain embodiments, the cell also expresses achemokine receptor. In certain embodiments, the chemokine receptor isCXCR4 or CCR5. In certain embodiments, the HIV nucleic acid encodesgp160, gp120, or gp41.

In embodiments of the invention where the methods determine whether anHIV population is resistant to an HIV entry inhibitor, the HIVpopulation can be a heterogeneous population. In certain embodiments,the HIV population can be a homogenous population. Preferably, the HIVpopulation corresponds to the population of HIV infecting a subject. Forexample, by determining the log-sigmoid inhibition curve of many, e.g.,50, 100, or more, individual env isolates from the subject, theresistance or susceptibility of the HIV population infecting the subjectcan be determined.

In certain embodiments, the indicator nucleic acid comprises anindicator gene. In another embodiment of this invention, the indicatorgene is a luciferase gene.

In certain embodiments, the cell surface receptor is CD4. In certainembodiments, the cell surface receptor is a chemokine receptor. Incertain embodiments, the cell surface receptor is CXCR4 or CCR5. Incertain embodiments, the cell surface receptor is both CXCR4 and CCR5.

In certain embodiments, the subject is infected with the HIV-1 virus, ahepatitis virus (such as the HCV or HBV virus), or any other virus.

In certain embodiments, the nucleic acid comprises DNA encoding gp160,gp120, and/or gp41.

In certain embodiments, the viral expression vector comprises HIVnucleic acid.

In certain embodiments, the viral expression vector comprises an HIVgag-pol gene.

In certain embodiments, the viral expression vector comprises DNAencoding vif, vpr, tat, rev, vpu, and nef.

In certain embodiments, the cell is a mammalian cell. In certainembodiments, the mammalian cell is a human cell. In certain embodiments,the human cell is a human embryonic kidney cell. In certain embodiments,the human embryonic kidney cell is a 293 cell. In certain embodiments,the cell is a human T cell. In certain embodiments, the cell is a humanT cell leukemia cell line. In certain embodiments, the cell is aperipheral blood mononuclear cell. In certain embodiments, the cell isan astroglioma cell. In certain embodiments, the astroglioma cell is aU87 cell. In certain embodiments, the cell is a human osteosarcoma cell.In certain embodiments, the human osteosarcoma cell is an HT4 cell. Incertain embodiments, the cell is a cell that does not naturally expressCD4, CXCR4, and/or CCR5, but has been engineered to express one or moreof these receptors. In certain embodiments, the cell has been engineeredto express CD4. In certain embodiments, the cell has been engineered toexpress CXCR4. In certain embodiments, the cell has been engineered toexpress CCR5.

In certain embodiments, the HIV entry inhibitor binds to the cellsurface receptor. In certain embodiments, the compound is a ligand ofthe cell surface receptor. In certain embodiments, the compoundcomprises an antibody, or an antigen-binding fragment thereof. Incertain embodiments, the compound inhibits membrane fusion. In certainembodiments, the compound is a peptide, a peptidomimetic, an organicmolecule, or a synthetic compound. In certain embodiments, the compoundbinds the viral envelope protein.

In another aspect, the invention provides a method for determiningsusceptibility of a virus to a compound which inhibits viral cell entrywhich comprises: (a) obtaining nucleic acid encoding a viral envelopeprotein from a subject infected by the virus; (b) co-transfecting into afirst cell (i) the nucleic acid of step (a), and (ii) a viral expressionvector which lacks a nucleic acid encoding an envelope protein, andwhich comprises an indicator nucleic acid which produces a detectablesignal, such that the first cell produces viral particles comprising theenvelope protein encoded by the nucleic acid obtained from the subject;(c) contacting the viral particles produced in step (b) with a secondcell in the presence of the compound, wherein the second cell expressesa cell surface receptor to which the virus binds; (d) measuring theamount of signal produced by the second cell in order to determine theinfectivity of the viral particles; and (e) comparing the amount ofsignal measured in step (d) with the amount of signal produced in theabsence of the compound, wherein a reduced amount of signal measured inthe presence of the compound indicates that the virus is susceptible tothe compound.

The invention provides a method for determining whether a virus hasdeveloped resistance to an entry inhibitor which comprises: (a)determining whether a virus is resistant to an entry inhibitor accordinga method of the invention, wherein a nucleic acid encoding a viralenvelope protein is obtained from a subject at a first time; (b)determining whether a virus is resistant to an entry inhibitor accordinga method of the invention, wherein the nucleic acid encoding the viralenvelope protein is obtained from the subject at a later second time;and (c) comparing the susceptibilities determined in steps (a) and (b),wherein a decrease in susceptibility at the later second time indicatesthat the virus has developed reduced susceptibility or resistance to theentry inhibitor.

The invention provides for a method for identifying a mutation in avirus that confers resistance to a compound that inhibits viral entryinto a cell which comprises: (a) determining the nucleic acid sequenceor the amino acid sequence of the virus prior to any treatment of thevirus with the compound; (b) obtaining a virus resistant to thecompound; (c) determining the nucleic acid sequence or the amino acidsequence of the resistant virus from step (b); and (d) comparing thenucleic acid sequence or the amino acid sequences of steps (a) and (c),respectively, so as to identify the mutation in the virus that confersresistance to the compound.

In certain embodiments, the virus obtained in step (b) is the virus ofstep (a) grown in the presence of the compound until resistance isdeveloped.

In certain embodiments, the virus obtained in step (b) is isolated froma subject which has been undergoing treatment with the compound.

In certain embodiments, this invention further provides a means andmethod for discovering, optimizing and characterizing novel or new drugsthat target various defined and as yet undefined steps in the virusattachment and entry process.

In certain embodiments, this invention further provides a means andmethod for discovering, optimizing and characterizing HIV-1 vaccines(either preventative or therapeutic) that target various defined and asyet undefined steps in the virus attachment and entry process.

In certain embodiments, this invention provides a means and method foridentifying amino acid substitutions/mutations in HIV-1 envelopeproteins (gp41 and/or 120) that alter susceptibility to inhibitors ofvirus entry.

In certain embodiments, this invention further provides a means andmethod for determining HIV-1 envelope amino acid substitutions/mutationsthat are frequently observed, either alone or in combination, in virusesthat exhibit altered susceptibility to virus entry inhibitors.

In certain embodiments, this invention further provides a means andmethod for using virus entry inhibitor susceptibility to guide thetreatment of subjects failing antiretroviral drug treatment.

In certain embodiments, this invention further provides the means andmethods for using virus entry inhibitor susceptibility to guide thetreatment of subjects newly infected with HIV-1.

In another aspect, the methods comprise determining that a subject isinfected with an HIV that is resistant to an HIV entry inhibitoraccording to a method of the invention, then advising a medicalprofessional of the treatment option of administering to the subject atherapeutic regimen that does not include the HIV entry inhibitor. Incertain embodiments, the HIV entry inhibitor is SCH-C, SCH-D, UK-427857,or TNX-355. In certain embodiments, the entry inhibitor is selected fromthe group consisting of BMS-488,403, Pro-542, mAb B4, TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, TAK-220, Pro-140, and mAb004.In certain embodiments, the entry inhibitor is selected from the groupconsisting of TNX-355, UK-427,857, SCH-D, GW-873,140, AMD-11070, andTAK-220. In certain embodiments, the entry inhibitor is selected fromthe group consisting of TNX-355, UK-427,857, SCH-D, GW-873,140, andTAK-220. In certain embodiments, the entry inhibitor is BMS-488,403. Incertain embodiments, the entry inhibitor is Pro-542. In certainembodiments, the entry inhibitor is mAb B4. In certain embodiments, theentry inhibitor is TNX-355. In certain embodiments, the entry inhibitoris UK-427,857. In certain embodiments, the entry inhibitor is SCH-D. Incertain embodiments, the entry inhibitor is GW-873,140. In certainembodiments, the entry inhibitor is AMD-11070. In certain embodiments,the entry inhibitor is TAK-220. In certain embodiments, the entryinhibitor is Pro-140. In certain embodiments, the entry inhibitor ismAb004.

In another aspect, the methods comprise determining that a subject isinfected with an HIV that is resistant to an HIV entry inhibitoraccording to a method of the invention, then advising a medicalprofessional not to treat the subject with the HIV entry inhibitor. Incertain embodiments, the HIV entry inhibitor is SCH-C, SCH-D, UK-427857,or TNX-355. In certain embodiments, the entry inhibitor is selected fromthe group consisting of BMS-488,403, Pro-542, mAb B4, TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, TAK-220, Pro-140, and mAb004.In certain embodiments, the entry inhibitor is selected from the groupconsisting of TNX-355, UK-427,857, SCH-D, GW-873,140, AMD-11070, andTAK-220. In certain embodiments, the entry inhibitor is selected fromthe group consisting of TNX-355, UK-427,857, SCH-D, GW-873,140, andTAK-220. In certain embodiments, the entry inhibitor is BMS-488,403. Incertain embodiments, the entry inhibitor is Pro-542. In certainembodiments, the entry inhibitor is mAb B4. In certain embodiments, theentry inhibitor is TNX-355. In certain embodiments, the entry inhibitoris UK-427,857. In certain embodiments, the entry inhibitor is SCH-D. Incertain embodiments, the entry inhibitor is GW-873,140. In certainembodiments, the entry inhibitor is AMD-11070. In certain embodiments,the entry inhibitor is TAK-220. In certain embodiments, the entryinhibitor is Pro-140. In certain embodiments, the entry inhibitor ismAb004.

In still another aspect, the methods comprise determining that a subjectis infected with an HIV that is resistant to an HIV entry inhibitoraccording to a method of the invention, and administering to the subjecta combination of anti-HIV agents that does not comprise the HIV entryinhibitor. In certain embodiments, the HIV entry inhibitor is SCH-C,SCH-D, UK-427857, or TNX-355. In certain embodiments, the entryinhibitor is selected from the group consisting of BMS-488,403, Pro-542,mAb B4, TNX-355, UK-427,857, SCH-D, GW-873,140, AMD-11070, TAK-220,Pro-140, and mAb004. In certain embodiments, the entry inhibitor isselected from the group consisting of TNX-355, UK-427,857, SCH-D,GW-873,140, AMD-11070, and TAK-220. In certain embodiments, the entryinhibitor is selected from the group consisting of TNX-355, UK-427,857,SCH-D, GW-873,140, and TAK-220. In certain embodiments, the entryinhibitor is BMS-488,403. In certain embodiments, the entry inhibitor isPro-542. In certain embodiments, the entry inhibitor is mAb B4. Incertain embodiments, the entry inhibitor is TNX-355. In certainembodiments, the entry inhibitor is UK-427,857. In certain embodiments,the entry inhibitor is SCH-D. In certain embodiments, the entryinhibitor is GW-873,140. In certain embodiments, the entry inhibitor isAMD-11070. In certain embodiments, the entry inhibitor is TAK-220. Incertain embodiments, the entry inhibitor is Pro-140. In certainembodiments, the entry inhibitor is mAb004.

In still another aspect, the methods comprise determining that a subjectis infected with an HIV that is hypersusceptible to an HIV entryinhibitor according to a method of the invention, then advising amedical professional of the treatment option of administering to thesubject a combination of anti-HIV agents that does not comprise aneffective amount of the HIV entry inhibitor. In certain embodiments, theHIV entry inhibitor is SCH-C, SCH-D, UK-427857, or TNX-355. In certainembodiments, the entry inhibitor is selected from the group consistingof BMS-488,403, Pro-542, mAb B4, TNX-355, UK-427,857, SCH-D, GW-873,140,AMD-11070, TAK-220, Pro-140, and mAb004. In certain embodiments, theentry inhibitor is selected from the group consisting of TNX-355,UK-427,857, SCH-D, GW-873,140, AMD-11070, and TAK-220. In certainembodiments, the entry inhibitor is selected from the group consistingof TNX-355, UK-427,857, SCH-D, GW-873,140, and TAK-220. In certainembodiments, the entry inhibitor is BMS-488,403. In certain embodiments,the entry inhibitor is Pro-542. In certain embodiments, the entryinhibitor is mAb B4. In certain embodiments, the entry inhibitor isTNX-355. In certain embodiments, the entry inhibitor is UK-427,857. Incertain embodiments, the entry inhibitor is SCH-D. In certainembodiments, the entry inhibitor is GW-873,140. In certain embodiments,the entry inhibitor is AMD-11070. In certain embodiments, the entryinhibitor is TAK-220. In certain embodiments, the entry inhibitor isPro-140. In certain embodiments, the entry inhibitor is mAb004.

In still another aspect, the methods comprise determining whether asubject is infected with an HIV that is resistant to an HIV entryinhibitor according to a method of the invention at a first time, thendetermining whether the subject remains infected with an HIV that isresistant to an HIV entry inhibitor according to a method of theinvention at a later second time. In other embodiments, the methodscomprise determining whether a subject is infected with an HIV that isresistant to an HIV entry inhibitor according to a method of theinvention at a first time, then determining whether the subject isinfected with an HIV that is no longer resistant to an HIV entryinhibitor according to a method of the invention at a later second time.

6.2 Measuring Resistance to an Entry Inhibitor

In this section, methods for performing single-round infectivity assaysassessing the ability of a virus to enter a cell are described. Thesemethods can be used, for example, to determine data points for plottinga log-sigmoid curve to assess whether an HIV is resistant to an entryinhibitor. In one embodiment, an envelope expression vector capable ofexpressing HIV-1 envelope proteins in transfected cells was constructed.A related expression vectors have been described, including a plasmid(pAmphoEnv) constructed to express amphotropic murine leukemia virus(A-MLV) envelope protein as described in U.S. Pat. No. 5,837,464 and(Petropoulos et al., 2000). The pAmphoEnv vector uses the immediateearly gene promoter of human cytomegalovirus (CMV) and the SV40polyadenlyation signal sequence to produce A-MLV envelope mRNA intransfected cells. The pAmphoEnv plasmid is modified by deleting theA-MLV envelope gene and introducing restriction enzyme cleavage sitesthat can enable the insertion of viral envelope fragments derived from avariety of isolates, such as HIV-1. In the case of, HIV-1, the envelopeopen reading frame spans approximately 2,600 nucleotides and encodes theenvelope polyprotein, gp160. The gp160 polyprotein is cleaved by acellular furin-like protease to produce two subunits, gp41 transmembraneprotein and gp120 surface protein. HIV-1 envelope expression vectors canbe constructed in stages as follows:

6.2.1 Replacing the A-MLV Envelope Nucleic Acid Sequences from theEnvelope Expression Vector (pAmphoEnv) with a Multiple Cloning SitePolylinker:

The A-MLV envelope nucleic acid sequences can be deleted from thepAmphoEnv vector by restriction enzyme digestion. The digested vectorcan be re-circularized by ligation to a duplex oligonucleotidepolylinker containing four unique internal restriction sites (a, b, c,d) for insertion of envelope sequences. The ligation reaction can beused to transform Escherichia Coli and molecular clones containing thecorrect polylinker sequence can be identified and confirmed byrestriction mapping and DNA sequencing, respectively. The introductionof multiple unique cloning sites into the vector can facilitate theinsertion of HIV-1 envelope sequences. Restriction sites within thepolylinker can be chosen based on their infrequent occurrence in HIV-1envelope sequences (LANL HIV-1 database). This vector can be referred toas pCX. The functionality of the pCX vector can be demonstrated byinserting a reporter gene or indicator nucleic acid, such as fireflyluciferase, into the pCX multiple cloning site and measuring a signalfrom the indicator nucleic acid or reporter gene activity in transfectedcells. As used herein, “indicator nucleic acid” refers to a nucleic acidencoding a protein, DNA or RNA structure that either directly or througha reaction gives rise to a measurable or noticeable signal, e.g. coloror light of measurable wavelength, or generation of a specific DNA orRNA structure used as an indicator which could be amplified using anyone of a number of quantitative amplification assays.

6.2.2 Inserting Viral Envelope Sequences into the PCX EnvelopeExpression Vector:

Using mutagenic primers for PCR amplification, viral envelope fragmentsare generated that contain two unique restriction sites (a, b and c, d,respectively) adjacent to the initiation and termination codons of, forexample, the HIV-1 envelope open reading frame. Introduction of twounique restriction sites at each end of the envelope open reading framecan improve chances of cloning HIV-1 envelope fragments harboringinternal restriction sites for any one of the enzymes found in themultiple cloning site of the pCX vector.

In the case of HIV-1, two well-characterized molecular clones of HIV-1with known differences in the envelope gene, NL4-3 (asyncytium-inducing, T-cell tropic, laboratory strain) and JR-CSF (anon-syncytium-inducing, macrophage-tropic, primary isolate) can be usedas-template for PCR amplification. The .about.2,600 nucleotideamplification products can be digested with two restriction enzymes(each enzyme cleaving at one end of the fragment; e.g. a and c or b andd) and subsequently inserted into the pCX vector by ligation andtransformation of Escherichia Coli. Molecular clones containing theappropriate envelope sequences can be identified by restriction mappingand confirmed by DNA sequencing. The resulting plasmids, pHIVenv (NL4-3)and pHIVenv (JR-CSF), can be used to express HIV-1 envelope proteins intransfected cells (FIG. 1A). The functionality of the envelopeexpression vectors, such as the pHIVenv vectors, can be demonstrated bymeasuring viral envelope synthesis in transfected cells (Western Blot),and by their ability to pseudotype envelope deficient retrovirusvectors. High titer virus stocks using the human embryonic kidney 293cell line has been demonstrated (Petropoulos et al., 2000), however thepresent invention is not restricted to those cell lines. Other suitablecell lines used as a first cell for transfection of nucleic acidobtained from the subject encoding a viral envelope protein include, byway an example and not as limitation to the present invention, 5.25;HOX; U87; MT2; PM1; CEM; etc. The cell line optimally will be engineeredto express one or more co-receptors.

6.2.3 Modifying the pCX hector to Improve the Efficiency of CloningViral Envelope Sequences:

To improve the cloning efficiency of viral envelope fragments, the pCXexpression vector can be modified by inserting a bacterial killer genecassette (e.g. control of cell death b gene (ccdB) or a member of thehok-killer gene family) under the control of the Escherichia Coli lacpromoter into the multiple cloning site (the et al., 1990; Bernard andCouturier, 1992; Bernard et al., 1993). This modified vector is referredto as pCXccdB. Transcription of the ccdB killer gene is repressed inbacterial strains that express the lac i repressor, such as JM109. Thisor an equivalent strain can be used to propagate plasmids carrying theccdB killer gene that are under the control of the lac promoter.Conversely, in this system bacterial strains that do not over expressthe lac i^(q) repressor, such as DH5α and Top 10, cannot maintainplasmids that express the ccdB gene. Transformants can be killed due tothe ccdB activity. DH5a and ToplO cells can be purchased from severalvendors (Life Technologies or Invitrogen). Using this selective cloningapproach, the parental expression vector is propagated in a lac i^(q)bacterial strain. The vector is digested with two restriction enzymesthat both remove the ccdB gene cassette, and, in the case of HIV-1, arecompatible with the insertion of HIV-1 envelope sequences (a, b, c, d).Following ligation of the vector and envelope fragments, a strain ofbacteria lacking laciq is transformed. Once transformed,bacteria-containing plasmids in which the viral envelope inserts havereplaced the ccdB killer gene can grow. Bacteria containing plasmidsthat retain or reconstitute the ccdB killer gene can not survive. Inthis way, the population of transformed bacteria is enriched forplasmids that contain viral envelope inserts, but is lacking in theparental vector containing the ccdB gene. The construction of thepCXccdB vector is not essential to practice the present invention, butit is expected to significantly improve the efficiency of cloning HIV-1envelope sequences derived from subject samples; thus, the probabilityof maintaining the heterogeneity of viral sequences can be improved. Thestructure of the pCXccdB vector can be confirmed by restriction mappingand DNA sequencing.

6.2.4 Inserting Viral Envelope Sequences into the pCXccdB ExpressionVector:

The functionality of the pCXccdB vector can be evaluated by setting upligation reactions containing viral envelope sequences and incompletelydigested pCXccdB vector DNA. Following bacterial transformation, plasmidDNA can be prepared from individual bacterial clones and analyzed byrestriction digestion for the presence of viral envelope fragments andthe absence of ccdB sequences. The feasibility of this approach istested by amplifying the envelope region from a total of 13 availableHIV-1 clones (pCRII-91U5005.11, pCRII-91006.10, pCRII-92US657.1,pCRII-92U5711.14, pCRII-91U5712.4, pCRII-92U5714.1, pCRII-91 HT652.11,pCRII-92BR020.4, pCRI-91 HT651.1A, pCRII-92HT593.1, pCRII-92HT594.10,pCRII-92HT596.4, pCRII-92HT599.24), obtainable through the AIDS researchreagent reference program (ARRRP), Rockville, Md. Each fragment can beinserted into pCXccdB and the structure of the resulting pHIVenvexpression vectors can be confirmed by restriction mapping and/or DNAsequencing. The functionality of each pHIVenv vector can be demonstratedby measuring HIV-1 envelope protein synthesis in transfected cells(Western Blot), and by their ability to pseudotype envelope-deficientretrovirus vectors.

6.3 Construction of a Bio-Safe Viral Expression Vector ComprisingIndicator Nucleic Acid in Place of the Encoding the Envelope Protein.

A bio-safe viral vector is constructed to evaluate inhibitors of viralentry according to similar means and methods as described in U.S. Pat.No. 5,837,464 and Petropoulos et al., 2000 used to evaluate inhibitorsof PR and RT. The viral expression vector of the present invention canbe co-transfected into cells together with the envelope expressionvectors (described above) to produce high titer virus stocks. Such virusstocks can be evaluated for susceptibility to inhibitors of virus entry,including antiviral drugs and neutralizing antibodies. In the case ofHIV-1, the viral expression vector can be generated from NL4-3, awell-characterized infectious molecular clone of HIV-1. The 5′ longterminal repeat (LTR) which controls viral gene expression can bemodified so that transcription of the viral genes in transfected cellsis driven by the CMV immediate early promoter (Naviaux et al., 1996).Most of the envelope gene can be deleted, but important control elementssuch as the rev responsive element (RRE) and accessory protein codingregions, (rev, tat) are retained. In place of the deleted envelopesequences, an indicator nucleic acid, such as a firefly luciferasereporter gene cassette that is under the control of CMVpromoter-enhancer sequences (FIGS. 1B and 3) is inserted. Virusinfection can be monitored by measuring luciferase activity in infectedcells. It is conceivable, although unlikely, that inter-plasmidrecombination between the retroviral vector and, for example, thepHIVenv sequences in transfected cells may lead to the generation ofinfectious HIV-1. In effort to generate a biosafe vector, introductionof several genetic alterations in the HIV genome can be made. Forexample, deletion of most of the envelope gene, while retaining theimportant control sequence, RRE, and also deletion of thetranscriptional enhancer sequences in the U3 region of the 3′ LTR of thevector (FIG. 2) can be accomplished. During the replication of theretroviral genome, the U3 region located at the 3′ end of the virusgenome serves as the template for the U3 region of the 5′ LTR of theprovirus in infected cells. Such proviruses lack the strong promoterelement in the U3 region of the 5′ LTR and thus are unable to produceretroviral RNA in infected cells. This self-inactivating (SIN) strategyhas been used successfully for several retroviral vector systems,including HIV-1 (Hwang et al., 1997; Miyoshi et al, 1998). In the assayof the present invention, viral gene expression is not required ininfected cells because virus infection is measured by a detectablesignal produced by the indicator nucleic acid, such as the production ofluciferase activity, driven by its own separate promoter (FIG. 1B).Deletion of envelope sequences and the transcriptional enhancer region(U3) can be accomplished by standard molecular cloning procedures, andeach deletion can be verified by DNA sequence analysis.

Functionality of this vector, for example in the case of HIV-1,designated pHIVlucΔU3, can be demonstrated by co-transfection of 293cells with the pHIVenv vector described above. Efficienttranscomplementation of viral proteins produced by both vectors in thetransfected cells can lead to the production of viral particles. Virusparticles can be harvested from the culture supernatants and analyzed byWestern-blotting. Virus titers can be quantitated by routineapplications of either p24 ELISA, quantitative PCR or TaqMan assays.

It is not necessary to produce a self-inactivating viral expressionvector to carry out the present invention, but it is desirable toimprove assay reproducibility and biosafety.

6.4 Identification of Suitable Cell Lines which Express Receptors andCo-receptors and Support Viral Infection.

Different mammalian cell lines that have been described previously andare known to support infection of a particular virus can be evaluated.As discussed herein for one embodiment relating to HIV-1, the assay canbe performed by (a) co-transfecting a first cell with pHIVenv andpHIVlucΔU3, (b) harvesting virus after transfection, (c) using thisvirus to infect a second cell, both in the presence and absence of virusentry inhibitors, and (d) measuring luciferase production in infectedcells.

Table 1 lists representative examples of such cell lines evaluated forHIV-1 infection, including the cell line and its associatedreceptor/co-receptor. Several of these cell lines can be obtained frompublic cell repositories.

Viral particles harvested from transfected 293 cell cultures can be usedto infect a variety of different cell lines. In the case of HIV-1, thepHIVlucΔU3 vector contains deletions in the envelope gene and the U3promoter-enhancer as described above, therefore infection of apermissive cell line with virus particles produced by this vector isrestricted to a single round of replication. This includes (a) virusattachment and entry, mediated by the viral envelope proteins, producedin trans by the pHIVenv vector as described, (b) the conversion ofsingle stranded viral RNA into double stranded DNA by RT, and (c)integration of viral DNA into the host cell genome (provirus formation).The active transcription of viral genes by RNA polymerase II thatnormally occurs in infected cells following proviral integration can berestricted by deleting essential viral promoter-enhancer sequences inthe pHIVlucΔU3 vector. However, this restriction can not interfere withluciferase gene expression in infected cells since this gene is drivenindependently of viral gene expression using an internal CMV promoter(FIG. 1B). The amount of luciferase activity produced followinginfection can be used as a measure of viral infectivity.

HIV-1 attachment and entry into host cells requires interaction with aprimary receptor (CD4) and one of several co-receptors, most often CCR5or CXCR4. Cell lines can be screened that are known to express variouscombinations of CD4, CCR5 and CXCR4. Specifically, cell lines listed inTable 1 that express (a) CD4 plus CCR5, (b) CD4 plus CXCR4, and (c) CD4plus CCR5 plus CXCR4 are evaluated. Cell lines that express the CD4receptor alone, or either the CCR5 or CXCR4 co-receptor alone, may serveas useful controls and can be used to evaluate HIV-1 isolates that donot require CD4 binding or that use co-receptors other than CCR5 andCXCR4. The principal criterion for judging cell line suitability can beinfectivity as measured by luciferase production (10⁴-10⁶ relative lightunits). In addition, cell lines can be evaluated based on growth rates,viability, stability and other parameters as deemed necessary. Celllines can be selected that are easy to maintain and for example, producelarge amounts of luciferase activity following infection, which can beinfected by different envelope receptor tropisms, e.g. CD4/CXCR4 andCD4/CCR5. Additional well-characterized cell lines that support, forexample, HIV replication and express the HIV-1 receptor and co-receptors(e.g. CEM-NKr-CCR5; release category a) are available through publicrepositories such as the ARRRP.

Further, cell lines can be enhanced using standard procedures, such aspromoting infection by the addition of polybrene to cells (Porter etal., 1998). For example, in the case of HIV, other potential cell linescan be identified for use with the present invention by infection withHIV-1 laboratory strains and comparing the recombinant virus infectivitytiters to those obtained with infectious HIV-1, or by transfecting cellsdirectly with the viral expression plasmids described herein, andscoring for virus production. Accumulation of viral transcripts can bechecked by using a quantitative RT-PCR assay. Cell lines suitable forother viruses can be identified in a similar manner.

The present invention can optimize assay conditions and allow forhigh-throughput testing of subject samples using automation. Samplepreparation methods can be optimized to efficiently capture viralgenomic and envelope RNAs. RT-PCR conditions can be optimized to enableamplification of patient-derived viral envelope sequences, such as HIV-1envelope sequences (.about.2,600 base pairs) at low viral loads(.about.500 copies per ml).

6.5 Demonstration of the Utility of the Assay

The utility of the assay of the present invention is demonstrated by theresults achieved from: (1) testing for dose-dependent inhibition ofviral entry in the presence of well-characterized inhibitors; and the(2) testing for dose-dependent inhibition of infection in the presenceof well-characterized HIV-1 neutralizing antibodies.

The following applications for the virus entry assay of the presentinvention were

i) detecting inhibition of HIV-1 replication by inhibitors of virusattachment and entry—(including fusion, receptor and co-receptorinhibitors);

ii) measuring changes in susceptibility to HIV-1 attachment and entryinhibitors;

iii) detecting neutralization activity of antibodies generated inresponse to vaccines targeted against HIV-1 envelope proteins; and

iv) determining co-receptor tropism.

In certain embodiments, the assay can be performed by (a)co-transfecting a first cell with pHIVenv and pHIVlucΔU3 vectors, (b)harvesting virus approximately 48 h after transfection, (c) using thisvirus to infect a second cell, both in the presence and absence of virusentry inhibitors and (d) measuring luciferase production approximately48-72 hr. after infection. Dose-dependent inhibition of HIV-1replication can be evaluated against a wide range of virus entryinhibitor concentrations using a 96-well format. The appropriateconcentration range can be determined empirically for each inhibitor.The data can be plotted as the percent inhibition of luciferase activityvs. drug concentration (log₁₀). Data analysis can be performed usingcomputer software Inhibition curves can be used to determine 50%inhibitory concentrations (IC₅₀) for specific drugs or antibodies.

Envelope proteins derived from a variety of well-characterized HIV-1isolates are evaluated using pHIVenv vectors constructed as describedabove. To define envelope co-receptor tropism, in the case of HIV-1,infection using cells expressing CD4 plus CXCR4 and CD4 plus CCR5 isevaluated as described above. A wide variety of compounds that are knownto inhibit HIV-1 entry (Table 2), including non-specific agents such assulfonated polyanions (dextran sulfate and heparin) can be used with theassay of the present invention. Chemokines such as Rantes and SDF-1, thenatural ligands for the CCR5 and CXCR4 chemokine receptors, respectively(see Alkhatib et al., 1996; Bleul et al., 1996) are also suitable foruse with the present invention. Further, virus entry inhibitors such asT-20 and T1249 (Trimeris, Inc.), PRO542 (Progenics), TNX 355 (Tanox)were used to evaluate utility of the assay of the present invention.

Drug toxicity in target cells are evaluated using standard viability orcytotoxicity assays (e.g. dye exclusion, MTS, ATP).

HIV-1 mutants exhibiting reduced susceptibility to the fusion inhibitorT20 (Rimsky et al., 1998) and the genetic determinants (mutations) thatenable these viruses to replicate in the presence of drug map within theenvelope protein (gp41-TM) have been described. To demonstrate that theassay of the present invention is capable of measuring changes in drugsusceptibility (i.e. resistance), (a) pHIVenv vectors are generated thatcarry these mutant envelope genes, (b) first cells are co-transfectedusing these vectors and the pHIVlucΔU3 vector, (c) viruses bearing thesemutant envelope proteins are harvested, and (d) the viruses are testedfor infectivity in the presence of T20. Reduced drug susceptibility toT20 is evaluated by comparing the IC₅₀ of viruses bearing mutantenveloped proteins to those that lack the defined drug resistancemutations. Viruses bearing envelope proteins with drug resistancemutations can exhibit higher IC₅₀ values than viruses bearing envelopeproteins that lack drug resistance mutations, i.e. inhibition canrequire a higher drug concentration. Drug resistance mutations can beintroduced into envelope expression vectors (pHIVenv) using standardsite directed mutagenesis techniques according to standard protocols(Petropoulos et al, 2000; Ziermann et al., 2000)

It is widely accepted that effective vaccines that protect against HIV-1infection should elicit a strong humoral immune response characterizedby broadly cross-reactive neutralizing antibodies. Consequently, theserum of vaccinated individuals is routinely evaluated for the presenceof high titer neutralizing antibodies targeted against the immunogen.Most recently, using the HIV-1/simian immunodeficiency virus (SIV)chimeric virus macaque model (SHIV), Mascola and colleagues have shownthat passive transfer of such neutralizing antibodies led to reducedviral load after mucosal challenge (Mascola et al., 2000). The assay ofthe present invention can be used to rapidly and reliably determine theviral neutralizing activity of antibodies generated in response tovaccines targeting envelope antigens, such HIV-1 envelope antigens. Forexample, the assay of the present invention can (a) generate pHIVenvvectors that express a variety of well-characterized envelope proteins,(b) co-transfect a first cell using these vectors and the pHIVlucΔU3vector, (c) harvest viruses and incubate with serial dilutions ofantibody preparations or vaccine serum (d) test these viruses forinfectivity in a second cell. Data analysis and IC₅₀ determinations canbe performed as described previously and in the literature. In the caseof HIV-1, viruses can be selected to represent different HIV-1 geneticbackgrounds (e.g. Glade A, B, C, D, E, F), different cell andco-receptor tropisms (macrophage/CCR5, T-cell/CXCR4), and differentenvelope properties (syncytium and non-syncytium inducing, laboratoryadapted growth or primary isolate) (Table 2). It can be beneficial toprepare stocks of a defined titer from each virus to optimize assaysensitivity and reproducibility by using a virus input of approximately20-100 TCID₅₀/well and making adjustments as necessary. Antibodypreparations can be selected based on previously documentedneutralization properties, either functional, such as their ability toneutralize primary isolates, or physical, such as their ability to bindspecific gp120 or gp41 epitopes (Table 2). The performance of the assayof the present invention can be judged against the activity of thesewell-characterized antibody reagents in conventional virusneutralization assays as described in the scientific literature. Serumfrom a broadly representative group of HIV-1 infected individuals can beused to establish an appropriate range of serum dilutions that canmaximize assay sensitivity, yet minimize cytotoxicity. Cytoxicity can beevaluated using standard viability or cytotoxicity assays (e.g. dyeexclusion, MTS, ATP).

6.6 Computer-Implemented Methods and Compositions for DeterminingWhether a Virus is Resistant to an Entry Inhibitor

In another aspect, the present invention provides computer-implementedmethods for determining whether an HIV is resistant to an entryinhibitor. In such embodiments, the methods of the invention are adaptedto take advantage of the processing power of modem computers. One ofskill in the art can readily adapt the methods in such a manner.Therefore, in certain embodiments, the invention provides acomputer-implemented method for determining whether an HIV is resistantto an entry inhibitor comprising inputting phenotypic information into amemory system of a computer, wherein the phenotypic informationcomprises data points representing the ability of the HIV to enter acell in the presence of varying concentrations of the HIV entryinhibitor; inputting a correlation between the inability of the HIVentry inhibitor to completely inhibit entry by the HIV and resistance ofthe HIV to the HIV entry inhibitor, and determining whether the HIV isresistant to the entry inhibitor.

In certain embodiments, the computer-implemented method for determiningwhether an HIV is resistant to an entry inhibitor comprises inputtingphenotypic information into a memory system of a computer, wherein thephenotypic information comprises log-sigmoid inhibition curve comprisingdata points that measure entry of the HIV into a cell in the presence ofvarying concentrations of the HIV entry inhibitor; inputting alog-sigmoid inhibition curve for a reference HIV; and comparing thelog-sigmoid curve of step (a) to the log-sigmoid curve of the referenceHIV, thereby determining whether the HIV is resistant to the entryinhibitor. In certain embodiments, a decrease in the maximum inhibitionpercentage observed for the HIV relative to that observed for thereference HIV indicates that the HIV is resistant to the HIV entryinhibitor.

In other aspects, the invention provides a computer-implemented methodfor determining whether an HIV population is resistant to an entryinhibitor that comprises inputting phenotypic information into a memorysystem of a computer, wherein the phenotypic information comprises datapoints representing the ability of the HIV population to enter a cell inthe presence of varying concentrations of the HIV entry inhibitor;inputting a correlation between the inability of the HIV entry inhibitorto completely inhibit entry by the HIV population and resistance of theHIV population to the HIV entry inhibitor, and determining whether theHIV population is resistant to the entry inhibitor.

In certain embodiments, the computer-implemented method for determiningwhether an HIV population is resistant to an entry inhibitor comprisesinputting phenotypic information into a memory system of a computer,wherein the phenotypic information comprises log-sigmoid inhibitioncurve comprising data points that measure entry of the HIV populationinto a cell in the presence of varying concentrations of the HIV entryinhibitor; inputting a log-sigmoid inhibition curve for a reference HIVpopulation; and comparing the log-sigmoid curve of step (a) to thelog-sigmoid curve of the reference HIV population, thereby determiningwhether the HIV population is resistant to the entry inhibitor. Incertain embodiments, a decrease in the maximum inhibition percentageobserved for the HIV population relative to that observed for thereference HIV population indicates that the HIV population is resistantto the HIV entry inhibitor.

In certain embodiments, the methods further comprise displaying whetherthe HIV is resistant to an entry inhibitor on a display of the computer.In certain embodiments, the methods further comprise printing whetherthe HIV is resistant to an entry inhibitor.

In another aspect, the invention provides a tangible medium indicatingwhether an HIV is resistant to an entry inhibitor produced according toa method of the invention. In certain embodiments, the tangible mediumis a paper document. In certain embodiments, the tangible medium is acomputer-readable medium. In certain embodiments, the paper document isa printed document, e.g., a computer print-out. In still another aspect,the invention provides a computer-readable medium comprising dataindicating whether an HIV is resistant to an entry inhibitor producedaccording to a method of the invention.

In yet another aspect, the invention provides a computer-readable mediumthat comprises data indicating whether an HIV is resistant to an entryinhibitor produced according a method of the invention. In certainembodiments, the computer-readable medium is a random-access memory. Incertain embodiments, the computer-readable medium is a fixed disk. Incertain embodiments, the computer-readable medium is a floppy disk. Incertain embodiments, the computer-readable medium is a portable memorydevice, such as, e.g., a USB key or an iPod™.

In still another aspect, the invention provides an article ofmanufacture that comprises computer-readable instructions for performinga method of the invention. In certain embodiments, the article ofmanufacture is a random-access memory. In certain embodiments, thearticle of manufacture is a fixed disk. In certain embodiments, thearticle of manufacture is a floppy disk. In certain embodiments, thearticle of manufacture is a portable memory device, such as, e.g., a USBkey or an iPod™.

In yet another aspect, the invention provides a computer-readable mediumthat comprises data indicating whether an HIV is resistant to an entryinhibitor and computer-readable instructions for performing a method ofthe invention. In certain embodiments, the computer-readable medium is arandom-access memory. In certain embodiments, the computer-readablemedium is a fixed disk. In certain embodiments, the computer-readablemedium is a floppy disk. In certain embodiments, the computer-readablemedium is a portable memory device, such as, e.g., a USB key or aniPod™.

In yet another aspect, the invention provides a computer system that isconfigured to perform a method of the invention.

7. EXAMPLES

The following examples are presented to further illustrate and explainthe present invention and should not be taken as limiting in any regard.Certain of these experiments were also described in U.S. applicationSer. Nos. 09/874,475 and 10/077,027, each of which is incorporated byreference in its entirety.

7.1 Example 1 Measuring Phenotypic Drug Susceptibility to Inhibitors ofHIV-1 Entry

This example provides a means and method for accurately and reproduciblymeasuring susceptibility to inhibitors of HIV-1 attachment and entry(heretofore collectively referred to as entry). Based on this example,the means and method for measuring susceptibility to inhibitors of HIV-1entry can be adapted to other viruses, including, but not limited toother lentiviruses (e.g. HIV-2), other retroviruses (e.g. HTLV-1 and 2),hepadnaviruses (human hepatitis B virus), flaviviruses (human hepatitisC virus) and herpesviruses human cytomegalovirus). This example furtherprovides a means and method for measuring alterations (increases anddecreases) in susceptibility to entry inhibitors.

Measurements of entry inhibitor susceptibility are carried out usingadaptations of the means and methods for phenotypic drug susceptibilityand resistance tests described in U.S. Pat. No. 5,837,464 (InternationalPublication Number WO 97/27319) which is hereby incorporated byreference.

One vector, an example of the envelope expression vector, (pHIVenv) isdesigned to express the envelope polyprotein (gp160) encoded by subjectderived HIV envelope sequences (FIG. 1). Gp160 is subsequently cleavedby a cellular protease to generate the surface (gp120SU) andtransmembrane (gp41TM) subunits that comprise the envelope protein onthe surface of HIV-1 virus particles. A second vector, an example of theviral expression vector, (either pHIVIuc or pHIVlucΔU3) is designed toexpress genomic and subgenomic viral RNAs and all HIV proteins exceptthe envelope polyprotein (FIGS. 1A-1B).

In this application, patient-derived segment(s) correspond to the codingregion (.about.2,600 nucleotides) of the HIV-1 envelope polyprotein(gp160) and represent either (a) envelope sequences amplified by thereverse transcription-polymerase chain reaction method (RT-PCR) usingviral RNA isolated from virus derived from HIV-infected individuals, or(b) envelope sequences derived from molecular clones of HIV-1 thatcontain specific mutations introduced by site directed mutagenesis of aparental molecular clone (typically NL4-3).

Isolation of viral RNA was performed using standard procedures (e.g.RNAgents Total RNA Isolation System, Promega, Madison Wis. or RNAzol,Tel-Test, Friendswood, Tex.). The RT-PCR protocol was divided into twosteps. A retroviral reverse transcriptase [e.g. Superscript II(Invitrogen, Life Technologies) Moloney MuLV reverse transcriptase(Roche Molecular Systems, Inc., Branchburg, N.J.), or avianmyeloblastosis virus (AMV) reverse transcriptase, (Boehringer Mannheim,Indianapolis, Ind.)] was used to copy viral RNA into first strand cDNA.The cDNA was then amplified to high copy number using a thermostable DNApolymerase [e.g. Taq (Roche Molecular Systems, Inc., Branchburg, N.J.),Tth (Roche Molecular Systems, Inc., Branchburg, N.J.), PrimeZyme(isolated from Thermus brockianus, Biometra, Gottingen, Germany)] or acombination of thermostable polymerases as described for the performanceof “long PCR” (Barnes, W. M., (1994) Proc. Natl. Acad. Sci, USA 91,2216-2220) [e.g. Expand High Fidelity PCR System (Taq+Pwo), (BoehringerMannheim. Indianapolis, Ind.) OR GeneAmp XL PCR kit (Tth+Vent), (RocheMolecular Systems, Inc., Branchburg, N.J.), Advantage-2, (CloneTech).

Oligo-dT was used for reverse transcription of viral RNA into firststrand cDNA. Envelope PCR primers, forward primer Xho/Pin and reverseprimer Mlu/Xba (Table 3) were used to amplify the patient-derivedsegments. These primers are designed to amplify the .about.2,600nucleotide envelope gene encoding the gp160 envelope polyprotein, whileintroducing Xho I and Pin AI recognition sites at the 5′ end of the PCRamplification product, and Mlu I and Xba I sites at the 3′ end of thePCR amplification product.

Subject derived segments (2.5 kB envelope sequence amplificationproduct) were inserted into HIV-1 envelope expression vectors usingrestriction endonuclease digestion, DNA ligation and bacterialtransformation methods as described in U.S. Pat. No. 5,837,464(International Publication Number WO 97/27319), with minor adaptations.The .about.2,600 nucleotide amplification product was digested witheither Xho I or Pin AI at the 5′ end and either Mlu I or Xba I at the 3′end. The resulting digestion products were ligated, using DNA ligase,into the 5′ Xho I/Pin AI and 3′ Mlu I/Xba I sites of modified pCXAS orpCXAT expression vectors. The construction of the pCXAS and pCXATvectors has been described in U.S. Pat. No. 5,837,464 (InternationalPublication Number WO 97/27319)). Modified pCXAS and pCXAT vectorscontain a Pin Al restriction site in addition to the Xho I, Mlu I andXba I restriction sites that exist in pCXAS and pCXAT. The Pin AI sitewas introduced between the Xho I and Mlu I sites by site directedmutagenesis, such that the four sites are located 5′ to 3′ in thefollowing order; Xho I, Pin AI, Mlu I and Xba I. In a preferredembodiment, the 2,600 nucleotide amplification products were digestedwith Pin AI and Mlu I and ligated into the 5′ Pin Al site and the 3′ MluI site of the modified pCXAS expression vector. Ligation reactionproducts were used to transform E. coli. Following a 24-36 h incubationperiod at 30-37.degree. C., the expression vector plasmid DNA waspurified from the E. coli cultures. To ensure that expression vectorpreparations adequately represents the HIV quasi-species present in theserum of a given subject, many (>100) independent E. coli transformantswere pooled and used for the preparations of pHIVenv plasmid DNA.Vectors that are assembled in this manner for the purposes of expressingsubject virus derived envelope proteins are collectively referred to aspHIVenv (FIGS. 1 and 3).

The genomic HIV expression vectors pHIVluc and pHIVlucΔU3 are designedto transcribe HIV genomic RNA and subgenomic mRNAs and to express allHIV proteins except the envelope polyprotein (FIG. 1B). In thesevectors, a portion of the envelope gene has been deleted to accommodatea functional indicator gene cassette, in this case, “Firefly Luciferase”that is used to monitor the ability of the virus to replicate in thepresence or absence of anti-viral drugs. In pHIVlucΔU3, a portion of the3′ U3 region has been deleted to prevent transcription of viral RNAsfrom the 5′ LTR in infected cells.

Susceptibility assays for HIV-1 entry inhibitors were performed usingpackaging host cells consisting of the human embryonic kidney cell line293 (Cell Culture Facility, UC San Francisco, SF, Calif.) and targethost cells consisting of a human osteosarcoma (HOS) cell line expressingCD4 (HT4) plus CCR5, and CXCR4, or astrocytoma (U-87) cell linesexpressing either CD4 and CCR5 or CD4 and CXCR4.

Drug susceptibility testing was performed using pHIVenv and pHIVluc orpHIVlucΔU3. Pseudotyped HIV particles containing envelope proteinsencoded by the subject derived segment were produced by transfecting apackaging host cell (HEK 293) with resistance test vector DNA. Virusparticles were collected about 48 h after transfection and were used toinfect target cells (HT4/CCR5/CXCR4, or U-87/CD4/CXCR4, orU-87/CD4/CCR5) that express the HIV receptor (i.e. CD4) and co-receptors(i.e. CXCR4, CCR5). After infection (.about.72 h) the target cells arelysed and luciferase activity is measured. HIV must complete one roundof replication to successfully infect the target host cell and produceluciferase activity. The amount of luciferase activity detected in theinfected cells is used as a direct measure of “infectivity” (FIGS. 1 and2). If for any reason (e.g. lack of the appropriate receptor orco-receptor, inhibitory drug activity, neutralizing antibody binding),the virus is unable to enter the target cell, luciferase activity isdiminished. Drug susceptibility is assessed by comparing the infectivityin the absence of drug to infectivity in the presence of drug. Relativedrug susceptibility can be quantified by comparing the susceptibility ofthe “test” virus to the susceptibility of a well-characterized referencevirus (wildtype) derived from a molecular clone of HIV-1, for exampleNL4-3 or HXB2.

Packaging host cells were seeded in 10-cm-diameter dishes and weretransfected one day after plating with pHIVenv and pHIVluc orpHIVlucΔU3. Transfections were performed using a calcium-phosphateco-precipitation procedure. The cell culture media containing the DNAprecipitate was replaced with fresh medium, from one to 24 hours, aftertransfection. Cell culture media containing viral particles wastypically harvested 2 days after transfection and was passed through a0.45-mm filter. Before infection, target cells were plated in cellculture media. Entry inhibitor drugs were typically added to targetcells at the time of infection (one day prior to infection on occasion).Typically, 3 days after infection target cells were assayed forluciferase activity using the Steady-Glo reagent (Promega) and aluminometer.

7.2 Example 2 Identifying Envelope Amino Acid Substitutions/Mutationsthat Alter Susceptibility to Virus Entry Inhibitors

This example provides a means and method for identifying mutations inHIV-1 envelope that confer reduced susceptibility/resistance to virusentry inhibitors. This example also provides a means and method forquantifying the degree of reduced susceptibility to entry inhibitorsconferred by specific envelope mutations.

Envelope sequences derived from subject samples, or individual clonesderived from subject samples, or envelope sequences engineered by sitedirected mutagenesis to contain specific mutations, are tested in theentry assay to quantify drug susceptibility based on awell-characterized reference standard (e.g. NL4-3, HXB2).

In one embodiment, susceptibility to longitudinal subject samples(viruses collected from the same subject at different timepoints) isevaluated. For example, susceptibility to entry inhibitors is measuredprior to initiating therapy, before or after changes in drug treatment,or before or after changes in virologic (RNA copy number), immunologic(CD4⁺ T-cell counts), or clinical (opportunistic infection) markers ofdisease progression.

7.2.1 Genotypic Analysis of Subject HIV Samples

Envelope sequences representing subject sample pools, or clones derivedfrom subject pools, can be analyzed by any broadly available DNAsequencing methods. In one embodiment of the invention, subject HIVsample sequences are determined using viral RNA purification, RT/PCR anddideoxynucleotide chain terminator sequencing chemistry and capillarygel electrophoresis (Applied Biosystems, Foster City, Calif.). Envelopesequences of subject virus pools or clones are compared to referencesequences, other subject samples, or to a sample obtained from the samesubject prior to initiation of therapy, if available. The genotype isexamined for sequences that are different from the reference orpre-treatment sequence and correlated to differences in entry inhibitorsusceptibility.

7.2.2 Entry Inhibitor Susceptibility of Site Directed Mutants

Genotypic changes that correlate with changes in fitness are evaluatedby constructing envelope expression vectors (pHIVenv) containing thespecific mutation on a defined, drug susceptible, genetic background(e.g. NL4-3 reference strain). Mutations may be incorporated aloneand/or in combination with other mutations that are thought to modulatethe entry inhibitor susceptibility. Envelope mutations are introducedinto pHIVenv vectors using any of the broadly available methods forsite-directed mutagenesis. In certain embodiments the mega-primer PCRmethod for site-directed mutagenesis is used (Sarkar, G. and Summer, S.S., 1990). A pHIVenv vector containing a specific envelope mutation orgroup of mutations are tested using the virus entry assay described inExample 1. Drug susceptibility of the virus containing envelopemutations is compared to the drug susceptibility of a geneticallydefined drug susceptible virus that lacks the specific mutations underevaluation. Observed changes in entry inhibitor susceptibility areattributed to the specific mutations introduced into the pHIVenv vector.

7.3 Example 3 Measuring Susceptibility to Virus Entry Inhibitors toGuide Treatment Decisions

This example provides a means and method for using virus entry inhibitorsusceptibility to guide the treatment of HIV-1. This example furtherprovides a means and method for using virus entry inhibitorsusceptibility to guide the treatment of subjects that have receivedprevious antiretroviral treatment with a virus entry inhibitor. Thisinvention further provides the means and methods for using virus entryinhibitor susceptibility to guide the treatment of subjects that havenot received previous treatment with a virus entry inhibitor.

In one embodiment, the susceptibility of subject's viruses to virusentry inhibitors is used to guide the treatment of subjects failingantiretroviral regimens that include one or more virus entry inhibitors.Treatment failure (also referred to as virologic failure) is generallydefined as partially suppressive antiviral treatment resulting indetectable levels of virus, which is typically measured in the subjectplasma). Guidance may include, but is not limited to, (a) clarificationof available drug treatment options, (b) selection of more activetreatment regimens, (c) clarification of the etiology of rising viralload in treated subjects (i.e. poor adherence, drug resistance), and (d)reduction in the use of inactive and potentially toxic drugs. In thisembodiment, resistance test vectors are derived from a subject virussamples and tested for susceptibility to various virus entry inhibitorsusing the phenotypic virus entry assay. Virus entry inhibitors mayinclude, but are not limited to, fusion inhibitors (e.g. T-20, T-1249),co-receptors antagonists (AMD3100, AMD8664, TAK779, PRO542, andpeperidinlyl butane compounds) and CD4 antagonists (MAb 5A8).Appropriate treatment decisions are based on the results of the virusentry assay (e.g. see FIG. 4B) and additional relevant laboratory testresults and clinical information.

In another embodiment, the susceptibility of subject's viruses to virusentry inhibitors is used to guide the treatment of subjects that havenot been previously treated with antiretroviral regimens that includeone or more virus entry inhibitors. Guidance may include, but is notlimited to, (a) clarification of available drug treatment options, (b)selection of more active treatment regimens, (c) clarification of thebaseline susceptibility to virus entry inhibitors, and (d) reduction inthe use of inactive and potentially toxic drugs. Determining baselinesusceptibility of virus entry inhibitors in treatment naive subjects isimportant for two reasons. First, the natural susceptibility of virusesto entry inhibitors can vary widely (e.g. see FIG. 4A). Second, theincreased use of virus entry inhibitors will undoubtedly result in thegeneration of drug resistant variants that can be transmitted to newlyinfected individuals. In this embodiment, resistance test vectors arederived from a subject virus samples and tested for susceptibility tovarious virus entry inhibitors using the phenotypic virus entry assay.Virus entry inhibitors may include, but are not limited to, fusioninhibitors (e.g. T-20, T-1249), co-receptors antagonist (e.g. AMD3100,AMD8664, TAK-355, PRO542, and peperidin-lyl butane compounds) and CD4antagonists (MAb 5A8). Appropriate treatment decisions are based on theresults of the virus entry assay and additional relevant laboratory testresults and clinical information.

7.4 Example 4 Determining Resistance to HIV Entry Inhibitors

This example provides means and methods for determining whether aparticular HIV or population of HIV is resistant or susceptible to anentry inhibitor. These means and methods are useful, for example, toguide therapeutic decisions in treatment subjects infected with HIV,whether newly infected or failing treatment, and for screening compoundsto identify compounds that will affect viruses resistant to other entryinhibitors. Other uses of such methods will be apparent to those ofskill in the art.

The methods generally rely on determining phenotypic drug susceptibilityof an HIV or an HIV population using, for example, the phenotypic drugsusceptibility assays presented in Example 1. However, any suchsusceptibility assay known by one of skill in the art can in principlebe used in the methods for determining phenotypic drug susceptibility.Drug susceptibility can be plotted, for example, as percent inhibitionversus log₁₀ drug concentration and defined based on, for example, theIC₅₀ and percent inhibition at the highest drug concentration. Suchpercent inhibition observed at the highest drug concentration for areference HIV, such as, for example, SF2, is the maximum percentinhibition (max % inhibition). In this example, the phenotypic drugsusceptibility assay of Example 1 was used to determine a log-sigmoidcurve showing the resistance of mutant HIV to SCH-C, TNX-355, T-20, andAMD-3100.

Resistance to certain of these entry inhibitors manifests withcompetitive kinetics, i.e., the inhibitors compete with viral proteinsfor access to viral or cellular components that mediate virus entry.Such entry inhibitors include, but are not limited to, T-20 andAMD-3100, discussed above. Viruses that are resistant to such entryinhibitors exhibit an increased IC₅₀ relative to susceptible viruses.See FIG. 5, demonstrating that T-20- and AMD-3100-resistant HIV exhibitincreased IC₅₀ for inhibition of entry by these compounds relative to asensitive reference virus, and FIG. 7, that such increased IC₅₀ relativeto sensitive viruses can be observed in mixed populations of viruses.

Resistance to certain of these entry inhibitors manifests withnon-competitive kinetics, i.e., the inhibitors do not directly competewith viral proteins for access to viral or cellular components thatmediate entry, but instead alter the conformation of the cellularcomponent and/or viral protein in a manner that disrupts the interactionbetween such proteins and components. Such entry inhibitors include, butare not limited to, UK-427857, SCH-C, SCH-D, UK-427857, and TNX-355.Viruses that are resistant to such non-competitive entry inhibitorsexhibit reduced maximum percentages of inhibition relative tosusceptible viruses. See FIG. 6, demonstrating that viruses resistant toSCH-C and TNX-355 exhibit reduced maximum percentages of inhibitionrelative to sensitive viruses, FIG. 8, demonstrating that virusesresistant to UK-427,857 exhibit reduced maximum percentages ofinhibition relative to sensitive viruses, and FIG. 7, demonstrating thatsuch reduced maximum percentages of inhibition relative to sensitiveviruses can be observed in mixed populations of viruses. Such virusesresistant to non-competitive entry inhibitors can also be detectedbecause such viruses cannot be completely inhibited with highconcentrations of the inhibitor. That is, no matter how much inhibitoris added to the assay mixture, entry of viral particles expressing theresistant envelope proteins could be detected.

Finally, the reductions in maximum percentage of inhibition observed forresistant virus were reproducible. FIG. 9 shows the maximum percentageof inhibition determined as described above for particular viralisolates repeated 9 times. As shown in FIG. 9, the maximum percentage ofinhibition observed for a particular isolate clustered well, indicatingthat the assays for determining the maximum percentage inhibition arereproducible.

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TABLE 1 Cells Cell Receptor 5.25 CXCR4, CD4, CCR5 (not expressed well)BONZO 5.25.Luc4.M7 CD4, CCR5, BONZO HOS.CD4.CCR5 CD4, CCR5 HOS.CD4.CXCR4CD4, CXCR4 HOS.CD4 CD4, low level expression of CCR5 and CXCR4 HOS HT4R5 GFP wt CD4, CXCR4, CCR5 HOS.CD4.CCR5.GFP.M7#6* CD4, CXCR4, CCR5P4.CCR5 CD4, CXCR4, CCR5 U87.CD4 CD4 U87.CD4 R5 CD4, CCR5 U87.CD4 X4CD4, CXCR4 MT2 CD4, CXCR4 MT4 CD4, CXCR4 PM1 CD4, CXCR4, CCR5 CEM NKrCCR5 CD4, CXCR4, CCR5

TABLE 2 Representative viruses and reagents Viruses Envelope^(a) Source89.6, SF2 R5-X4/SI/B ARRRP^(B) 92BR014, 92US076 R5-X4/SI/B ARRRP JR-CSF,91US005 R5/NSI/B ARRRP 91US054 SI/B ARRRP NL43, MN, ELI X4/B ARRRP92HT599 X4 ARRRP 92UG031 R5/NSI/A ARRRP (IN-HOUSE) 92TH014, 92TH026R5/NSI/B ARRRP (IN-HOUSE) 92BR025, 93MW959 R5/SI/C ARRRP (IN-HOUSE)92UG035 R5/NSI/D ARRRP (IN-HOUSE) 92TH022, 92TH023 R5/NSI/E ARRRP(IN-HOUSE) 93BR020 R5-X4/SI/F ARRRP (IN-HOUSE) Antibodies Epitope SOURCEMabs 2F5, 1577 gp41 TM ARRRP Mabs IG1b12, 2G12, 17b, gp120 SU ARRRP 48DNeutralization sera #2, HIV- Polyclonal ARRRP IG Entry inhibitors TargetSource CD4-IG gp120 SU Genentech CD4-IGG2 gp120 SU Adarc SCD4 SigmaProgenics T20 (DP178) gp4l TM Trimeris Rantes, MIP1a/b CCRS SIGMA/ARRRPSDF1a/b CXCR4 SIGMA/ARRRP AMD 3100 CXCR4 AnorMed Dextran sulfate,Heparin Non-specific Sigma ^(a)R5 (CCRS co-receptor), X4 (CXCR4co-receptor) SI (syncytium inducing), NSI (non-syncytium inducing), A,B, C, D, E, F (envelope clade designation) ^(b)AIDS Research andReference Reagent Program

TABLE 3 Primers Tested for the Amplification of HIV Envelope RT PRIMERSPrimer 1 5′-GGA GCA TTT ACA AGC AGC AAC ACA GC-3′ Primer 25′-TTC CAG TCA VAC CTC AGG TAC-3′ Primer 35′-AGA CCA ATG ACT TAY AAG G-3′ 5′ PCR PRIMERS Primer 45′-GGG CTC GAG ACC GGT CAG TGG CAA TGA GAG TGA AG-3′ Primer 55′-GGG CTC GAG ACC GGT GAG CAG AAG ACA GTG GCA ATG A-3′ Primer 65′-GGG CTC GAG ACC GGT GAG CAG AAG ACA GTG GCA ATG-3′ 3′ PCR PRIMERSPrimer 7 5′-GGG TCT AGA ACG CGT TGC CAC CCA TCT TAT AGC AA-3′ Primer 85′-GGG TCT AGA ACG CGT CCA CTT GCC ACC CAT BTT ATA GC-3′ Primer 95′-GGG TCT AGA ACG CGT CCA CTT GCC ACC CAT BTT A-3′ Primer 10 5′-GAT GGT CTA AGA CGC TGT TCA ATA TCC CTG CCT AAC TC-3′

What is claimed is:
 1. A method for treating a patient with a virusentry inhibitor, comprising: (a) obtaining or having obtained abiological sample from a patient comprising a population of humanimmunodeficiency virus (HIV) viral particles; (b) contacting or havingcontacted a plurality of HIV viral particles from the population with acell in the presence of varying concentrations of an entry inhibitor,wherein the cell expresses a cell surface receptor to which the HIVviral particles bind, and wherein the cell or HIV viral particlecomprises an indicator nucleic acid that produces a detectable signalwhen the HIV viral particle enters into the cell; (c) collecting orhaving collected data points that measure entry of the HIV populationinto the cell by measuring the detectable signal produced by the cell inthe presence of varying concentrations of the virus entry inhibitor; (d)generating or having generated a log-sigmoid inhibition curve with thecollected data points; (e) determining or having determined a baselinesusceptibility value to the virus entry inhibitor for the HIV populationas measured by a maximum inhibition percentage or ability to completelyinhibit the HIV population; (f) comparing or having compared thebaseline susceptibility value of step (e) with a baseline susceptibilityvalue for a reference population of HIV viral particles, wherein abaseline susceptibility value of step (e) that is equal to or higherthan the baseline susceptibility value for the reference HIV populationindicates that the potential course of treatment with the virus entryinhibitor is likely to be effective for the patient; and (g) treatingthe patient with the virus entry inhibitor if the baselinesusceptibility value of step (e) that is equal to or higher than thebaseline susceptibility value for the reference HIV population.
 2. Themethod of claim 1, wherein the virus entry inhibitor is SCH-C, SCH-D,UK-427857, SDFIa/b, AMD3100, TNX-355, T-20.
 3. The method of claim 1,wherein the reference HIV population is an HXB2, NL4-3, or SF2 HIVpopulation.
 4. The method of claim 1, wherein each of the plurality ofHIV viral particles comprise: (i) a viral expression vector that lacks anucleic acid encoding a viral envelope protein, but which comprises anindicator nucleic acid that produces a detectable signal when introducedinto the cell, and (ii) a viral envelope protein encoded by a nucleicacid of the HIV population.
 5. The method of claim 1, wherein each ofthe HIV viral particles comprises the same viral envelope protein. 6.The method of claim 1, wherein the indicator nucleic acid encodesluciferase.
 7. The method of claim 1, wherein the plurality of viralparticles are produced by co-transfecting into a cell (i) a plurality ofnucleic acids each encoding a viral envelope protein of the HIVpopulation, and (ii) a viral expression vector lacking a nucleic acidencoding an envelope protein, wherein the vector comprises an indicatornucleic acid that produces a detectable signal.
 8. The method of claim1, wherein the cell surface receptor is at least one of CD4, CXCR4, andCCR5.
 9. The method of claim 1, wherein the HIV nucleic acid encodes thesurface envelope glycoprotein (gp120), the transmembrane envelopeglycoprotein (gp41), the envelope polyprotein (gp16), or both gp120 andgp41.
 10. A method for treating a patient having an HIV population,comprising: (a) contacting or having contacted a plurality of HIV viralparticles from the HIV population with a cell in the presence of an HIVentry inhibitor, wherein the HIV entry inhibitor is an antibody orfragment thereof, wherein the cell expresses a cell surface receptor towhich the viral particles bind, and wherein the cell or the viralparticle comprises an indicator nucleic acid that produces a detectablesignal when the viral particle enters into the cell, (b) collecting orhaving collected data points that measure entry of the HIV populationinto the cell by measuring the amount of the detectable signal producedby the cell in the presence of varying concentrations of the HIV entryinhibitor; (c) generating or having generated a log-sigmoid inhibitioncurve comprising the data points showing the maximum inhibitionpercentage for the HIV population; (d) comparing or having compared themaximum inhibition percentage of the inhibition curve of step (c) to themaximum inhibition percentage of a log-sigmoid inhibition curve for areference HIV population; (e) determining that the HIV population hasreduced susceptibility to the HIV entry inhibitor if the maximuminhibition percentage observed for the HIV population is lower than themaximum inhibition percentage observed for the reference HIV populationor if the entry of the HIV population cannot be completely inhibited bythe HIV entry inhibitor; and (f) treating the patient with an effectiveamount of an inhibitor other than the HIV entry inhibitor if the maximuminhibition percentage observed for the HIV population is lower than themaximum inhibition percentage observed for the reference HIV populationor if the entry of the HIV population cannot be completely inhibited bythe HIV entry inhibitor.
 11. The method of claim 10, wherein thereference HIV population is an HXB2, NL4-3, or SF2 population.
 12. Themethod of claim 10, wherein the HIV population is from a subjectinfected with HIV.
 13. The method of claim 10, wherein each of theplurality of viral particles comprises: (i) a viral expression vectorthat lacks a nucleic acid encoding a viral envelope protein, but whichcomprises an indicator nucleic acid that produces a detectable signalwhen introduced into the cell, and (ii) a viral envelope protein encodedby a nucleic acid of the HIV population.
 14. The method of claim 10,wherein the indicator nucleic acid encodes luciferase.
 15. The method ofclaim 10, wherein the plurality of viral particles are produced byco-transfecting into a cell: (i) a plurality of nucleic acids, eachencoding a viral envelope protein of the HIV population, and (ii) aviral expression vector lacking a nucleic acid encoding an envelopeprotein, wherein the vector comprises an indicator nucleic acid thatproduces a detectable signal.
 16. The method of claim 10, wherein thecell surface receptor is at least one of CD4, CXCR4, and CCR5.
 17. Themethod of claim 10, wherein the HIV viral particle comprises gp160, orboth gp120 and gp41.
 18. The method of claim 10, wherein the cellcomprises the indicator nucleic acid that produces a detectable signalwhen the viral particles enter into the cell.
 19. The method of claim 1,wherein the entry inhibitor is an antibody.
 20. The method of claim 19,wherein the antibody binds to the HIV envelope surface glycoprotein(gp120).
 21. The method of claim 19, wherein the antibody binds to theCD4 binding site or the variable region 3 (V3) of the envelope surfaceglycoprotein.
 22. The method of claim 19, wherein the antibody binds tothe HIV envelope transmembrane glycoprotein (gp41).
 23. The method ofclaim 22, wherein the antibody binds to the membrane proximal externalregion (MPER) of the HIV envelope transmembrane glycoprotein.
 24. Themethod of claim 10, wherein the entry inhibitor is an antibody.
 25. Themethod of claim 24, wherein the antibody binds to the HIV envelopesurface glycoprotein (gp120).
 26. The method of claim 25, wherein theantibody binds to the CD4 binding site or the variable region 3 (V3) ofthe HIV envelope surface glycoprotein.
 27. The method of claim 24,wherein the antibody binds to the HIV envelope transmembraneglycoprotein (gp41).
 28. The method of claim 27, wherein the antibodybinds to the membrane proximal external region (MPER) of the HIVenvelope transmembrane glycoprotein.
 29. The method of claim 1, whereinthe cell comprises the indicator nucleic acid that produces a detectablesignal when the viral particles enter into the cell.